Here's my reasoning for how it's possible for it to be low-diff.
Let's say back in Arlong, Zoro was a 10 while Mihawk was a 100. Zoro went all out while Mihawk used about as much effort as a 25. Zoro got low diffed because Mihawk used a level that's above Zoro by 15. Fast forward to present day, assuming Mihawk hasn't gotten any stronger, Zoro could be about a 60 now. If they fought now, Mihawk could use the effort of a level 75. Even though Zoro shot up in strength, there's still enough between them for Mihawk to produce the same result as last time, all Zoro making tons of progress means Mihawk has to use a ton more effort.