I'm not a massive expert, so I may be totally off-base, but:
I think it's reasonable for there to be some variability in growth rates just due to genetics, especially at an early stage.
The soil may have different levels of nutrients, or the soil may have uneven levels of compactness, meaning some of the roots may take a bit longer to branch out.
Lastly, I'm not sure I'm seeing this correctly, but it looks like you have multiple seedlings stuck together in each pot?
I'm not a massive expert, so I may be totally off-base, but:
What seeds are you using, btw?