The PIE *septm should have changed to Pre-PG *sefθen, by the Grimm's Law. Then, by the Verner's Law, it should have changed to *sebθen. Why did the *θ disappear for the word to develop into "sieben" in German and into "seven" ('b' regularly changes to 'v' in English between two vowels) in English?
I'd expect it to have developed into German **siebden ('th' changes to 'd' in German regularly) and English **sevthen or **sebthen.