First of all, I gotta put in a disclaimer here to happy up the Mods: I am not an agent or employee of any TPMS company and this is in no way a solicitation. This is for information only.
That said, I talked to Chuck and Deb who own MinderResearch (Tire Minder TPMS). They confirmed that the one-hour span of time before there is an indication of a dropped sensor signal is, as Mike at TST said, industry wide. Chuck noted that in his own testing another brand (not Tire Minder) never EVER gave an indication of a lost signal and the monitor continued to have a phantom reading off the memory in the monitor.
Many everyday things can cause signal interference, such as the GPS noted by a poster previously in this thread. Chuck and Deb said they take signal loss and interference issues seriously and they now include a repeater/booster with every TPMS system they sell. (TST sells them separately.)
One suggestion we made was that there be some kind of alarm or indication that there has been a lost signal much sooner than an hour. Perhaps even an adjustable setting where the user could set the limit for the monitor to be in "looking for it" mode (5 minutes, 10 minutes, whatever) before the system alarms.
Granted it would mean much more frequent alarming, but, yeah, I'd buy a system like that!