Flip this around. You can say the same thing about internal things/values - there are 'many different internal options'. So they too are arbitrary, in much the same sense you might view external behaviors being arbitrary. But again, doesn't mean a lot of thought isn't put into the why behind why you've chosen one internal 'way' of viewing things. It's just that the J's are going to concentrate efforts on honing external things and making that precise/solid, and P's are going to focus efforts on internal solidity.
(urg... I'm kinda making this up as I go so I'm sure it isn't entirely accurate)
Basically it's just two sides of the same coin (well, sort of ) -- and thank heavens everyone isn't solely internally perceiving without any external judgment, or solely internal judging without any external perceiving. And... it's why J's and P's can learn & gain from one another.