Part of the reason you are not allowed to buy a tkt when you are on standby is to prevent an attempt to stop people blocking inventory.
I'm in no way suggesting you did this
this is the process.
1 Standby for a flight
2 book a full fare fully refundable tkt this will remove one seat from the inventory
3 No-show for your full fare tkt At cutoff time your standby will be successful because there was a no show
4 refund your full fare tkt
This would result in you having been given a standby seat and the airline lost the opportunity to sell the seat
Again, I am not saying this was your plan but it's probably written into the S/W or a policy of the airline