"when Walmart mileage did not go to Schedule C, the refund is higher"

Huh? Are they crossing a tax bracket?

Mileage allowance as a business deduction on the Sched C will reduce business net, making their taxable amount less, which would increase a refund. If it's not on Sched C, are you putting it elsewhere?

"Is it okay for people to ignore some expenses in general?"

If you can't substantiate it, you shouldn't be reporting it. No log, no entry, no deduction.