When 3E was released, there was no cap... hence the solar with his 5 attacks.
Core classes only go up to level 20, so there was, in theory, no need to provide rules for PCs with higher than +20 BAB... except, of course, that a 3E Clr-20 could pull off a BAB of +32, temporarily...
So, until the ELH came out (and with it the 4 attack cap, which should have retro-fitted the Solar as well - was it ever stated that natural HD ignore the cap, or did people just make that up to fit the Solar?), that Cleric would get 7 iterative attacks while his spells lasted...
-Hyp.