3 things:

1) Level of gunners

In addition to the amount of experts and vets, higher level gunners provide higher stats with the same amount of vets. In most cases, you can reach acc cap before lvl 100 with B/V/E 10/10 gunners before level 100.

2) FCS

Many EBBs have the equivalent of BB5 FCS, which makes their spread better than stock BB2/3s.

3) Guns themselves

Some guns simply have bettter spreads, like IJN 20" dual guns (used on IJN EBB4.5/5/5.5) and UK 14" dual/quad guns (used mostly on POW and sometimes Nelson). Some guns are helpless, like USN Sodak 16" trips.