1st and 10?

[nelson_28602]

We had this play tonight and I would like your comments.
1st and 10 for A at B's 45. A runs the ball for a 5 yard gain to the 40 where after the ball is dead, both A and B committ personal fouls. B is assessed 15 yards, then A is assessed 15 yards to bring the ball back to the 40. The chains are reset at the 40, 1st and 10. My question is, "Why were the chains reset?" A did not reach the original line to gain, i. e. 35. Any thoughts or comments?
Thanks for any help.