There are many variables that go into addressing the question at hand, so the much dreaded answer is, "it depends".
several things that stand out in my mind:
- maintenance - one school of thought might be that more heads is going to mean more maintenance. That's not a black and white truth, but also depends on some other factors. I have over 100 sprinkler heads in our common areas in the subdivision that many have failed (due to the fact that small rotors were installed, which I learned from my last house never to use again). I also noticed that in our 1 acre's park they did what the last bidder did for your estimate. They installed as many of these crappy rotors as they could. I, on the other hand, went to the other end of the extreme when I installed my own sprinklers. I designed around the smallest number of heads I could get away with, and also max'd out each zone to the limit of gpm we're allowed to take (30gpm). I left no headroom, which may have been a mistake.
- overlap - 1 must ensure that there is proper overlap patterns with the placement and design of the heads. With odd-shaped yards, it requires more heads to get tangential-approximations of curves, etc. Or, you just overwater the boundaries which works too.
I'm sure there's quite a few other things I haven't mentioned.
My last thought is if the 5th bid seems odd, out of the norm, that's because it is. Why would a 5th quote differ so much from the norm? Could be the guy is a perfectionist like me, and wants to do things right. Or maybe he doesn't know what he's doing. Hard to say.