How many decimal places should a volume measurement in a beaker include?
It may have to do with the fact that it's a beaker, rather than some other kind of glassware.

First, beakers are poor vessels for volumetric measurements due to their large diameter/height ratios.  You have to add a lot of liquid, relatively speaking, in order to increase the height of the fill level by (say) a millimeter, and that means the uncertainty in the fill volume is large.

As well, beakers are usually not graduated precisely. Thus, even if you very very carefully fill the beaker to exactly the $15.5~\mathrm{mL}$ line with liquid, there's a very real chance that the line doesn't actually correspond exactly to a $\mathit{15.5~mL}$ fill volume.

So, unlike with a piece of glassware specifically made for accurate volumetric measurements, such as the burette mentioned by MaxW in a comment, it is likely not appropriate to assign more significant figures to a beaker measurement than are "available" in the graduation increment.