Is the gearshift indicator light in the instrument pod indicating a downshift to 4th when you press harder on the gas in 5th as you describe? If not, I think what you're experiencing is the torque converter "unlocking", which is normal if you apply light pressure to the accelerator at speed. Otherwise, if so, that's also normal if you're applying heavier pressure to the accelerator at speed.
Now, if you floor it at, say, 55MPH while in 5th, it will probably shift all the way down to 3rd for a moment, then up to 4th for a bit, then 5th, but by that time you'll be doing about 85-90MPH, so I'd let back up on the gas before it gets to that point.
(five-0)
As far as the gear ratios, 5th is indeed "overdrive", meaning that for every revolution completed by the axle, the engine driveshaft completes 0.692 of a revolution.
- SR