What Steve meant was that *if* the PC and router were in the same room
and in line-of-sight to each other, and *if* you still got a bad
signal, then external interference would be almost certainly the cause
of your problem.
As your PCs are on different floors to the access point, the problem
will be a combination of poor signal propagation and possibly
interference, too. A lot depends on the construction of the building.
As long as the PCs work OK (ie internet connection speed is good enough
for you), just ignore the meter displays.
--
Rob