Assalam Muhammad. There is a rule established since the middle ages when the Normans changed many phonological rules of the OE. One of the rule is : if a vowel is proceeded by a double consonant, the vowel is short. If the vowel is proceeded by a single consonant, it becomes longer or even takes a different phonological property.

E.g.: Later vs. Latter

Diner vs. Dinner In the case of Diner 'i' has a different sound namely /ai /

The length of English vowels depends or varies much according to

1- Their context.

2-The presence or absence of stress.