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?

What seeds are you using, btw?

I prefer GeoMastr / Geo-Tips, tbh.