I am standing on dirt wood is on slot 6 i am on slot 4 i press 6 it turns into dirt sometimes

I am standing on dirt wood is on slot 6 i press 6 it makes it dirt

it becomes dirt

1. place any block in a slot like wood in slot 4
2 stand on a block
3.go on slot 3 then 4 and it might turn to the block you are on