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.
