Often because one quote already includes more cost items (waiting time or tolls, for example) and the other doesn't. So always compare the breakdown, not just the total.
A careful quote should state all foreseeable costs; unexpected extras (such as a changed route) are something you discuss in advance with the coach company itself.
Yes, the price estimates shown include VAT.
Fill in your place of departure, your destination and your group, and compare the coach companies with their prices.