Maybe this will help:
100W isn’t much power. On level ground, it’s enough for you to cruise around at ~ 16 mph +/- (and this is true regardless of rider weight, because gravity isn’t a factor on the flats. So, a 150 lb rider @ 100 W may go 16 mph, and a 250 lb rider @ 100W will go a bit over 15 mph). In Zwift? On a flat course. This is on the slow side.
If you want to go faster on the flats? You need to shift into a higher gear, pedal faster, or both.
When you ride up a hill in Zwift with a controllable trainer (like yours), the trainer automatically applies more resistance. Assuming you don’t shift into a lower gear when this happens, your power output increases because you’re now turning the same gear against more resistance.
The reason you’re suddenly passing people on the way up has to do with your power-to-weight ratio (aka w/kg). Based on the W you’ve posting here, I’m guessing you are probably on the lighter side, weight-wise. So, as you start climbing and cranking out more watts, your w/kg increases. And, likely, it’s suddenly higher than those around you - which causes you to climb past them.
Let’s say you weigh 150 lb (68 kg).
At 100W on the flat, you’re at just under 1.5 w/kg and going ~ 16 mph.
When you’re climbing at 300W, you’re going at 4.4 w/kg (speed depends on the % grade)
Compare that to what happens with a larger rider who is 225 lb (102 kg)…
On the flat, at the same relative effort that you put in @ 1.5 W/kg? The larger rider is putting out: 102 kg * 1.5 w/kg = 153 W, which is good for ~ 18 mph - and so they ride away from you!
But when things go uphill… the larger rider has to sustain 448 W to match your effort.
Now, there are any number of reasons why people may appear to slow down disproportionately on climbs in Zwift. It may be that they’re heavier and don’t have the strength required to fly up hills, but I think it’s just as often the case that people are riding in ‘workout mode (ERG)’ and may be in the middle of a rest interval. Or they just chose to drop down into a really low gear and take it easy because they just finished a sprint. Whatever the case, you needn’t feel bad for climbing faster than them. There’s always plenty of yo-yoing that happens with riders in Zwift.
It’s all good.