Could be to do with the fact that the A10 was released one month previous or based on how many phones are out there so server space is used efficiently would be my guess. I doubt we'll ever know for sure.
Updates for specific models can be delayed or abandoned if found to be too buggy to be released. Read the One UI announcement which explains all this in greater detail.
Why is the S21 line the last of Samsungs flagships to be updated to the latest android security patch? The S20, Note 20 and even Note 10 line all got it first. It's just one of life's mysteries we may never know...
