It's not really "illegal" and I wouldn't even call it unexpected.  In real life, just about every product we buy comes in storage containers (boxes or bottles) which even though they are "full" when purchased can still have some more added.

The maximum capacity of storage bins in the current version (and for most versions before) is actually 105% of the listed capacity, so one bin that should hold 100 items can be over filled to 105 items, especially if there are multiple bots filling it at the same time.  This is similar to when the bots are in a "Repeat until Storage Full" loop, for which Storage Full is true when it is only 95% full.