Because the "high beam" isn't really a high beam - it's one bulb which acts as high/low (not to be confused with a dual filament incandescent bulb like on the MkIV Jetta's).
Located in the headlight unit is a flap which flips up when you turn (or flash) your highs on, allowing 100% of your light to be exposed. When the flap is down, and your low beams on, only maybe 50% of your light will shine through. Roll your window down and then flash the highs, you can hear the flap flipping up/down.
So the high beams are really the low beams, but that flap opens up fully to let more light out.
That's also why when you turn your high beams on at night, you don't get more light down low where the low beams were already shining, but rather more light up high (and down the road).
And yes, it would be ideal to change your bulbs at the same time due to temperature change.
Your stock bulbs are (or should be) OEM 4300k. As time goes on, they "heat up" in temperature, turning bluer. Granted, your lights aren't that old, so you will have to make the call on that one. It all depends on how many hours you have on the bulbs, and if your DRL's have been disabled. If they have been disabled, that dramatically decreases the time the hid's have been on - saving the life of the bulb/ballast.
Maybe go elsewhere (your local VW tuner) for bulbs. You should be able to get OEM Bosch bulbs from Germany for MUCH less than the dealer is charging you.