By default, scale bars (at least the default ones) round to 2 decimal places. This is almost never a good idea. I teach students to use neat, fractional units (e.g. 0.125, 0.25, 0.5, 1.0), which is also tricky to execute if you're new to ArcGIS. But once you get down to the eighth of anything, it rounds that 0.125 to 0.13, and this is terrible! It looks wrong and it is wrong.
In my opinion, there should never be rounding in a scale bar. If you're making a scale bar with values that have so many decimals that rounding is required, you should adjust the division values, not the rounding. Precision is important, and should be forwarded to the map reader when possible. The default scale bars usually come out with awkward units somewhat masked by rounding.
Novices won't know how to fix this (the element pane leaves much to be desired), and I also hate having to fumble through those settings every time. Sure enough, as much as I belabor this point to students, I get map after map with 0.13 in the scale bar, so making the default have NO rounding would be much better. Perhaps that's a selfish interest, though I also anticipate this will be more desirable among users, especially if a way to enforce fractional units by default as well (another scalebar "must" IMO - would you buy a ruler if its units were 0.17, 0.35, 0.70 ... ?).
