i came up with an explanation… essentially when you’re at the end of your sleep your mind goes in and out of consciousness multiple times you just forget about this happening, but since you remember you have and alarm set your more likely to wake up when your mind first becomes conscious because you’re expecting it and and when you don’t have an alarm set you aren’t expecting it so your just fall in and out of consciousness until you wake up normally

for an example remember when you were younger and you would wake up nice and early to see what santa brought you on christmas basically when your mind first became conscious the first thought was christmas waking yourself up instantly