I suspect it's the flip, that running or sprinting unencumbered are already far less efficient than walking. . .so the armor penalty to that efficiency is less dramatic, and less noticeable in tactical time.
i.e. if you have a compact car (high fuel efficiency) and an SUV (Low fuel efficiency). . .you weld on a trailer hitch and tow a 5 ton trailer.
The compact car fuel efficiency plummets, the SUV takes a smaller (percentage) loss in efficiency. . .because it's already inefficient to start.
i.e. walking in armor is much harder than walking without, because walking is a very efficiant means of moving. . .running in armor is harder, but takes a smaller (percentage) loss in efficiancy.
Of course, if you can walk 20 miles unencumbered, but only 10 miles armored, that has less of a dramatic tactical impact than being reduced from 100 yards sprinting distance to 75 yards sprinting distance. . .it's a 50% hit on walking which is double the 25% hit on sprinting, but the sprinting reduction is far more noticeable in a tactical round by round situation, since it'll take a crap load of rounds to walk to 10 miles and hit your limit, but might be able to sprint 75 yards in one round and crash into that limit.