In most Indo-European branches, the PIE sound *ḱ (a palatalized velar) evolved into something akin to a 'k' sound, while in some it evolved differently.
* Greek ἑπτά (heptá) and its cognates in other branches: Generally reflect a *k* sound in these related languages, leading to a "sept-" "septem" type sound.
* English *night* and its Germanic cognates: The PIE *ḱ in the root for "seven" underwent a sound change unique to Germanic languages known as the High German consonant shift, or a related shift depending on the specific Germanic language family. This involved a *ḱ* evolving not into a 'k' sound but becoming a fricative 'h' which resulted in the “night” spelling in English. This shift didn’t occur in Greek.
So, the key difference isn't just a simple spelling variation; it’s a fundamental difference in how the ancestral sound *ḱ evolved in the Proto-Germanic branch compared to the evolution in the Proto-Greek branch. This sound change distinguishes the Germanic "night" from the Greek "heptá" and their cognate forms in other families.