It is very common on the F series and B series units, as they have the same trans-axle assemblies.
With the unit running, the oil level drops below the dipstick tube opening, draining it. If the O-ring seals on the dipstick are sealing good, the dipstick tube is not vented, and the oil can not re-enter until the dipstick is removed. With it removed, oil can then enter the tube.
As stated, you never check a fluid level without withdrawing the dipstick, wiping it, reinserting it, removing it, and reading it.