I always recommend people use their own pressure gauge to check their own tyre pressures...Not a servo one.....
I have 2 seperate tyre pressure gauges at work, other than the tyre machine ones and the tyre pressures vary by 5 psi between the 2.....
One of them is a michelin one and the other is the old style servo ones.......
As for a tyre not sealing when fitted from new....The only times a tyre will leak from the bead is if the bead is torn, or some style of wheel weights cause a leak or the bead of the rim has a build up of crap that hasn't been cleaned off prior to fitting the tyre....If it's not loosing air constantly over a couple of days, I'd be putting it down to not being inflated correctly when fitted, or when it was being checked at the servo, the air chuck wasn't put on correctly initially, causing air to escape