I know HK follow Chinese script. Well... obviously. Their Cantonese words have direct a counterpart in Mandarin, like 你, is 'ni' in Mandarin and 'nei' in Cantonese. But our Cantonese, we say lei. Why are we so different?

Another example:

的
Mandarin: De
HK: Dik
Local: Gei

对
Mandarin: Dui
HK: Deoi
Local: Ngam
UPDATE:

Apparently I am wrong as pointed out by a ktard. These words have a direct counterpart in Mandarin, see answer 5.
的 is Dik
嘅 is Ge (correct)

对Is Duii
啱 is ngam (correct)
/K answers as simplied below:

1. Influenced by other dialects

2. The words we use are slangs, which HK also use. But we don't know the formal words, which HK knows and use

3. These are 'Lazy' pronunciation, as the language evolved.

4. KL more different from Hongkie. But Ipoh/Sabah closer to HK

5. The 'slangs' are actually found in Mandarin words. It is just that they are rarely used in Mandarin but have more meaning in Canto.
Additional info on corrupted Malaysian Cantonese:

http://www.usj.com.my/bulletin/upload/show...se-is-corrupted]http://www.usj.com.my/bulletin/upload/show...se-is-corrupted

https://chineselanguage.org/forums/viewtopic.php?f=2&t=654