netherrack breaking times inaccurate

so i FINALLY went from spawn to 11000, and i spawned over an ocean, which was dissapointing, but not the bug, although i suggest that if theres land nearby thats not underwater, thats where the portal should go. anyways, onto the bug. so i was not smart enough to bring a water bucket (i had 13 normal ones), so i had to get some. because of this, i placed blocks underwater, and i did netherrack because its easy to break and i have stacks of it (who dosent). anyway, when i broke the netherrack with my efficincy V diamond pickaxe, i noticed that it wouldnt break instantly. with diamond pickaxe efficincy II it says it can instant mine, but clearly it cant if efficincy V cant. it was like mining with an unenchanted stone pickaxe.