To be fair, he also didn't know it wasn't his final form

Next time on Dragon Ball Super: Freeza goes platinum.

Here's what's really confusing; his first three forms are actually forms he created to surpress his power, and his original final form is actually just his original form. On top of that, he developed two new, more powerful forms beyond his original form later. So, if Freeza were to fully power up from his weakest to strongest form, it would go fourth form, third form, second form, first form, fifth form, sixth form.