Has anyone had their ADHD medication quantity reduced because they collected it a few days early?

I've noticed that if I pick up my prescription a few days before I run out, the pharmacy seems to deduct tablets from the next supply. For example, I've received 28 tablets instead of 30, and another time 26 instead of 30.

We're talking about a difference of only a couple of days, so it feels quite punitive. Is this normal practice for ADHD medication in the UK, or should you receive the full quantity prescribed if my collection day is a few days off?