No expert in drilling wells, but information is out there.
Water Facts #6 Protecting Wells with Sanitary Well Caps ...
According to this, sanitary caps and grouting are used to prevent surface contamination, but grouting adds $500 - $1000 to the cost of a new well.
Going by that information but not knowing the specific sanitation regulations of your area, and the difference in price quoted, it would seem that a concrete pad would be part of the reason that bid was less than the others.
If the county sanitarian allows pads instead of grouting, it would be a way to cut corners on cost, but may lead to e-coli contamination of the water supply.
It appears that according to the study, concrete slabs are poor protection as they crack and allow contamination, but are cheaper to install, but provide minimum protection for the owner.
Existing wells are very difficult to grout, so it would be cheaper to grout the new well than to install the concrete pad and have to repair later.
Like I said, not my field so I am only guessing based on the PennState study.