I have made many mapbooks using data-driven pages. However I've encountered something strange and wanted to ask to see if anyone might know what's going on or has seen this before.
I have a scale bar in the title block and as I step through each page the scale bar moves slightly (0.0389 inches). I move it back but every time I try to export, the resulting pdf has the scale bar moved by 0.0389 on the first page, 0.0778 on the second, etc.
Seems like a bug, but maybe I'm missing something.
I am running into this on 10.5.1, so I don't believe it was automatically fixed. The scale bar works fine if it's anchored by its left side and allowed to adjust at the right end, but anchored on the right (also with its bottom-right Anchor Point selected in Properties > Size and Position tab > Position field) will eventually make it walk off the page.
You do not mention the version of ArcGIS for Desktop that you are using, but I am guessing that it is 10.0 because I recall an issue similar to this at that release, which was resolved by MapsurroundElement (arcpy.mapping) being enhanced at 10.1 (or possibly at one of the later service packs to 10.0) to handle the scale bar anchor point more predictably.
X and Y element positions are based on the element's anchor position, which is set via the Size and Position tab on the Elements Properties dialog box in ArcMap.
My recommendation would be to upgrade to the latest release of ArcGIS for Desktop, or if that is not an option, then ensure that you are using Service Pack 5 to ArcGIS Desktop 10.0.