Killing Creeper while attacking another mob gives extra health

Whenever I attack a zombie and a creeper explodes while I am attacking the zombie I am given extra yellow hearts. This is specifically in survival and I was able to recreate it after it happened for the first time. I do not know if it is important, but when recreating it and while it happened I was wearing a full set of iron armor and I was using an iron sword. I was able to perform this glitch two times, but was not able to recreate it for a third time.