I haven't ever grown camellias - :( - having never lived in a climate with a warm enough winter for them (& knowing I have neither the time nor patience to attempt growing one indoors), so I know very little about they whys and wherefores about how they bloom as they do (beyond knowing they like acidic soils in the same way that azaleas and rhododendrons do, even though they aren't in the heath family as azaleas & rhododendrons are).

As for roses (& this also probably applies to a fair number of other species in the rose family, apples for example), their genes often (USUALLY, probably) give the flowers not a specific number of petals that must be produced, but rather a range of petals that may be produced. With the wild rose species they typically have single flowers, and thus only five petals plus lots of stamens. However, some of the species (or naturally occurring hybrids, such as the White Rose of York) are semi-double (because some of the stamens have converted into petals), or have naturally occurring variants that are semi-double, or even double.

That genetic plasticity is what rose breeders play with, and sometimes the hybrids breeders create (all garden "varieties" bred by humans are hybrids of some sort or another) contain that plasticity within one plant, or even within one cluster of flowers.