ORLY?

Is that why my 26"x1.5" Schwalbe Marathons measure 1.62"? (That's a little over three millimeters difference)

The only tire in this household that measure anywhere close to the stated size are the 700x38 Specialized Nimbus tires on my girl's bike. Those measure 38.6mm.

With anything, tire width is going to depend largely on numerous factors. One big one being width of the rim they are put on.