You've exposed a combination of two things: high torque at low engine RPMs + high mechanical advantage => forward motion without requiring additional throttle (where "high" is defined to be "high enough to move the car").
There's another way to investigate the same problem that is likely easier on your clutch. First, get moving in low gear at a low but moving speed pointed up the hill in question (no clutch pedal required). Now, take your foot off the throttle: is the engine capable of moving the car up the hill at idle? If not, you'll slow and the engine will start to stall (that's the time to use the clutch pedal so you don't lug the engine).
Using this technique makes it a little more obvious what's going on:
At idle, your engine is putting out an effective minimum level of torque. This is what's necessary to spin the transmission and, thereby, rotate the wheels to move the vehicle.
Your transmission acts as a multiplicative coefficient on the torque output of the engine. In low gear, this coefficient is quite small (AKA a low gear ratio) so that many revolutions of the engine amount to only a single rotation of the wheels. This gives the engine + transmission combination a mechanical advantage, allowing the vehicle to do more work for less throttle (remember that idle is still not zero throttle).
So, using my 2004 WRX as an example:
At low revs, the turbo is barely moving. As a result, the engine is effectively producing only as much as a base model Impreza from that year (i.e., not much).
However, my first gear is a fairly low gear ratio. This means that I can get my car up a shallow slope into the garage from a dead stop without using the throttle pedal at all.
The end result is that, when I was teaching my son how to drive my car, I didn't have him use the throttle pedal at all for the first day. We were idling around in third gear without difficulty.