Wheels' Lug nuts loosened unexpectedly

Last month I rotated the wheels on the '06 Camry, from front
to back.  Yesterday I re-torqued them after 350 miles of driving
and five weeks.

All the lug nuts on the two rear wheels did not turn even
a bit; they were perfectly torqued.  But 4 out of 5 lug nuts on
the passenger-side front wheel turned a little bit, less than a
quarter turn--probably about an eighth of a turn (1/8 turn).

On the driver-side front wheel, 2 out of 5 lug nuts also turned
about 1/8 of a turn when I re-torqued them.

I know these 6 loosened lug nuts on the two front wheels
lost more than 5 foot-pounds of their original 76 foot-pound
torque setting because yesterday to re-torque them, I set my
torque wrench at 71 foot-pounds, which is 5 less than the
76 foot-pounds I torqued them to last month during the
5,000-mile wheel rotation.

Is this greater-than-5-foot-pound loosening of some of the
lug nuts on the front wheels something to be concerned
about?  I'd like to wait until the next 5,000-mile wheel rotation
before re-torquing the wheels again.  Is this advisable, or should
I check and re-torque the wheels more often?