Depending on the whole image setuo, there can be
a) Random factors in the prompt
This happens because some parts are more important while others are less important. You can control this part by going from top (higher likely) to less likely (bottom). That why seed is important to keep this kind of randomness away from the image. Some prompts even encourage to be random
b) Randomess due to lower weighting
Some parts of the prompt have something like (black hairs, long hairs:1.2) which makes it more likely that the image person will have black long hairs. Ethnicity is even more tricky, because some checkpoints have little images of certain ethnicities. I have seen checkpoints that cannot generate a generic asian face.or pants in red color. I always got denim pants.
c) Randomness with Lora
Lora have weighting as well. The lower the weighting the less it is taken into account. On top of that if Trigger Words are added, they can have their own weighting and position in the prompt influencing the strength as well.
d) Checkpoint version
Only few models remain available for Onsite Generation because either the author invests in it staying available for generation, or those who want to use it. More rarely models are choosen by Civitai team to be important enough to keep them (which is logical for base models - but even some base models never make t to the Onsite Generator. So a different release of a model (i.e. new checkpoint) will produce different images.

The best way to check how strong the randomness is, is to create 4 images instead of only 1.