I occasionally have an issue in Layout - If I pull a dimension between two points on the same horizontal plane, the dimension reads accurately. However, if I try to pull a horizontal dimension using points that are not on the same horizontal plane, the dimension reads as the diagonal distance between the two points no matter how I orient the dimension after initiating it. Attached is a screenshot for reference. This only happens occasionally, and I can’t figure out what setting configuration to change.
Have you tried pulling down or up further on the page?
This is possibly a consequence of the cutting plane not being exactly perpendicular to the treads. I have a feeling there are recent changes in the code that have altered the mathematical accuracy and inference sensitivity. If the dimension sees a distant point first it will latch on to it. I haven’t tried this but if you adjust the camera view in Layout you should see the connection points for the dimensions.
I was caught out by a similar thing a few months back that didn’t relate to the cut plane, the dimension was latching onto a node in a lower one of my stacked views. I now turn off any stacked views prior to dimensioning. Could that be a reason?
Those buttons in the Dimension Style panel move the text, not the measurement — the docs describe Horizontal/Vertical as positioning the text relative to the screen, and Aligned/Perpendicular as positioning it relative to the dimension line. So once a dimension is placed, nothing in that panel will change the number.
The number comes from what the dimension is connected to. So the thing worth checking is what it actually attached to, rather than how it’s oriented.
There’s a documented way to see that: double-click the dimension’s text to open its editing mode, and red dots appear at the connection points. That shows you where it’s really attached instead of where it looks attached. On a stair section in particular, a lot of candidate points sit almost on top of each other in the projection — two points can look level on the page and be nowhere near level in the model.
Worth confirming Object Snap is on as well, since dimensions only track geometry as it changes when it’s enabled (it is by default).
DGSketcher’s stacked-viewport experience would be the same mechanism — a connection landing somewhere you didn’t intend — but I wouldn’t assume that’s your cause. The red dots will tell you in your own file, which is quicker than disabling stacked views and re-dimensioning to find out.
And if the red dots turn out to be exactly where you wanted them, then this explanation doesn’t cover it and it’s something else — better to say that than send you down the wrong path.
When you double-click one of the bad dimensions, where do the red dots actually land?
Correction to my post above, before you go looking for something that may not be there.
The red dots I described are documented specifically for a dimension showing a red alert — one that has lost its connection. Yours isn’t in that state; it’s reporting a number, just not the one you want. So double-clicking may not give you red dots.
What is documented generally is the edit mode itself: double-click the dimension, or select it and press Enter, to enter edit mode and get at its connection points from there.
The substance of the answer doesn’t change — the measurement follows what the dimension is attached to, and the Dimension Style buttons only move the text. But I shouldn’t have implied the red dots show up on any dimension, and I’d rather flag that than have you conclude nothing’s wrong because you didn’t see them.
That’s interesting - I’ve often wondered if depth of view can effect a 2D dimension in Layout (obviously it shouldn’t) so I just did a quick experiment.
I drew this arrangement up in SketchUp, but I couldn’t get Layout to give me anything other than the correct 1414 dimension based on a 2D front view.
Having re-read your post I think I misunderstood your question 
If I want to get the true distance between two points that are not horizontally or vertically aligned, I hold down the Alt key while pulling the dimension. Maybe you are somehow activating that function.