Interesting thought, On those tracks I don't let the slapped or popped notes sustain maybe that's got something to do with it.  
 
Maybe that's why a lot of players who use slap a lot have a compressor limiter somewhere in the signal chain to even out their level.

I noticed that difference, as well. My experience that the volume change between finger style and slap often comes down to technique. About 25 years ago, I spent a lot of time with my 4 track cassette machine and I worked very hard on getting consistent levels regardless of style of playing. It paid off because I rarely need a compressor. However, other factors can come into play, such as tone settings, strings, etc. Unless you really like the tone that a compressor provides (which is a completely valid choice as it can sound very cool), I don't think it's the way to go. It's very much worth it to work out getting it all consistent because then you can switch back and forth for a few notes at a time and not have it sound weird. Eventually it becomes second nature. Of course, it's been a very long time since I did much slapping.... :-)
 
Sometimes pickup design can affect the volume discrepancies between finger and slap styles, but I have never had that issue with Alembic pickups.

Here are some youtube video links from my gig at the Simmer Down Festival, Birmingham, England in 2014
Series I Europa in action in all video's
 
Simmer Down 2014

 

 

 
And one from the Bedford River Fest

Here are some youtube video links from my gig at the Simmer Down Festival, Birmingham, England in 2014
Series I Europa in action in all video's
 
Simmer Down 2014

 

 

 
And one from the Bedford River Fest