Why is it that if I learn the pitch accent for one word, it is completely different when put in context with a sentence?

Sometimes, the pitch accent is the complete opposite when in a sentence vs as a single word.

I have a basic understanding of 頭高、中高、平板、尾高, but I can't find any clue in these as to why the pitch accent flips itself over depending on context of the sentence.

If anybody could help, I'd greatly appreciate it.