The Brown Bear seems to be equivalent to a fourth level character with D10 as hit dice. It has STR +4, and thanks to its 'level' a proficiency bonus of +2.
Based on those stats, its to-hit should be +6, not +5.
The often compared Dire Wolf (same level, about the same) has only STR +3, but still to-hit +5. So the proficiency bonus is not based on CR, as I first thought.
Is this a typo? Or just minor balancing applied without correcting the attributes?
Are there any official comments on this?
PS: The Brown Bear is just one example, this comes up multiple times in MM.