The positions (constellation) of the satellites are also important. The orbits of the GPS satellites are designed to minimize the incidence of poor constellations. (GLONASS may be better in the polar regions due to a higher orbital inclination.) Of course, local objects can block the line of sight to a satellite and reflections (multipath) can degrade the signals.
I also have no trouble receiving GPS signals in typical NE hiking locations with my 60CSx carried in the top of my pack. (Any of Garmin's "high sensitivity" GPSes should be similar.) I did have occasional trouble in similar conditions with earlier non-"high sensitivity" Garmin GPSes.
The GPS signal frequencies were chosen to minimize any weather effects. The L1 signal has a wavelength of ~19cm so airborne precipitation particles have essentially no effect because they are so much smaller than the wavelength.
The Garmin L1 signal is at 1575.42 MHz and the GLONASS L1 signals are centered on 1602 MHz (1598--1605MHz). The bandwidth of the Gilsson antenna is 17MHz (ie .1575.42 +- 8.5MHz)*. So the GLONASS signals are technically outside the antenna bandwidth, but your experiments suggest that it still works well enough.
*
http://www.gilsson.com/high_performance_gps.htm
A primary determinant of performance for a weak-signal receiver (eg a GPSr) is the signal-to-noise ratio (S/N). The primary purpose of the amplifier in an external antenna is to offset the losses in the cable. Both the GPS and the external antenna connect the physical antenna directly to a low-noise amplifier (LNA)--the antenna supplies the signal and the LNA the noise. If you used an external antenna without the built-in LNA, the S/N would be degraded by the cable losses (significant at these frequencies for typical cable lengths) between the antenna and the LNA in the GPSr. If there were no loss between the external antenna and the LNA in the GPSr, then there would be no need for an amplifier in the antenna. Too much gain in an external amplifier can actually degrade performance by overloading the GPSr.
True, however mounting an external antenna on top of your hat may be even better (unless your pack is unusually tall). Professional users often use a choke ring antenna (looks like a ~1ft diameter disk) on a pole to keep the antenna above both their head and pack.
I use an external antenna (similar to the Gilsson) on top of my hat when mapping a trail, but for hiking, just put the GPS in the top of my pack. Works well enough for my 60CSx and 62s (I have observed no problems with the recorded track) and no delicate cables to kink or snag. If I want the best available accuracy for a short period, all I have to do is take it out of my pack and hold it up over my head with the antenna oriented for best reception. (Best orientation depends on the type of antenna and how it is mounted inside the GPSr.)
Doug