RP Amount NOT Going down

TLDR; I GOT A FREE TRUNDLE SKIN???? (Totally Not Click Bait) By the end of this report/story, the most of you will think I'm stupid for telling this to Rito but I just want to be honest.... A few days ago a friend of mine bought me RP for my birthday (after the purchase i had 3485 RP) and I already started the calculations of what can I buy and all that. So I bought a Trundle skin (750) but the RP amount that I had didnt go down, so I figuered "oh its that bug again, it'll go down soon". After that, I've sent my friend a Nami skin (1350) and the RP amount that I had dropped to 2135 (which is 3485 - 1350), meaning the Trundle skin amount didnt go down from what I had. So I wanted to test my bug theory and see if I can purchase a legendary skin, I assumed it wont let me just like last time I saw this bugh happen. But it did, and after that skin I also bought a summoner icon. I re-did the calculations, and If you add up ALL the RP that I used, add whats left, and subtract the Trundle skin cost (750) you get 3485, which is what I started with. What happened here? Yours truly, GamingFist