It may depend on the cauliflower you are using (fresh/frozen). Some may be a little "sweeter", others not so much. In the frozen cauliflower packs that are posted online, there is no sugar in them, just cauliflower. So it's probably due to the cauliflower.

Cauliflower is a natural product and there are always variations. There are also countless factors that can influence the taste: Lots of sun, little sun, grown outside, grown in a greenhouse, cold snap at an early stage of growth, cold snap at a later stage, nutrients in the soil, too little water, enough water, too much water, different varieties... The list can be extended indefinitely.

Presumably because the frozen cauliflower is frozen directly on site, shortly after picking. Fresh cauliflower, on the other hand, has been lying around for several hours (or days) and has therefore already lost a lot of its nutrients (including vitamins) by the time it reaches your pot.