You're right that they're not the same quote. The components are material grade, square footage, edge profile, cutouts for sink and cooktop, tearout and disposal of the old top, and plumbing disconnect and reconnect, which some shops handle and some leave to you. Big box pricing usually looks lower because half of that is added later. Ask each one for a written scope with those items listed. When we did our
Quartz Countertops in Charleston, South Carolina with Perfect Stones the estimate came back within a day with the line items spelled out, which made comparison possible. Edge profile is where budgets quietly grow.