/jp/ - Otaku Culture

I've been through the guide reading about the long vowel sounds and it says that the vowel sounds of / i / e / are extended by い but when i'm going to watch the Tae Kim vid, he says the same shit but that nigga pronounces えい as /ei/ instead of an /ee/, けい as /kei/ instead of /kee/ and せい as /sei/ instead of /see/ which is fucking weird because at the end of his video lesson, he pronounces がくせい as "gakusee" and せんせい as "sensee" so my question is: Why does he pronounce えい as /sei/ but when he's going to say がくせい he pronounces it correctly which is "gakusee"?

Here's the link:
https://youtu.be/SLP1BxU6AR4?t=116