It's a quirk of this particular instance of the Second Conditional:
Zero Conditional: It's true every time - present tense in the condition, present tense in the result (If you heat water to 100 degrees, it boils)
First Conditional: The probable/possible future - present tense in the condition, future tense in the result (If we finish on time, I'll be home by 6pm)
Second Conditional: The improbable/imaginary present or future - past tense in the condition, would+infinitive in the result (If I won the lottery, I would buy a big house)
Third Conditional: The imaginary past that didn't happen - past perfect in the condition, would + present perfect in the result (If I had been there, I would have seen there)
Normally we never say "I were x" we'd say "I was x" (ie "I were at the cinema earlier, sounds incorrect). But for some reason, in this particular instance of the second conditional, we often say "if I was you". It's more of a grammatical quirk than a hard and fast rule.
At least that's how I teach it to my ESL students. I am not a linguistic expert though!