I just weighed two AP B1boxes that I have here and they were 99 and 103 grams.

So using the guide of 110 gr maximum it should leave you safe?

You're right!  They do vary a little - it might depend on how sensitive your scales are tho - I have 3 sets of scales and got a slightly different answer each time    so IMO when calculating postage for listing it's best to allow 110g - then when you actually pack something you might have a little leeway.