LZ77: why often possible to increase the match length to the left?

[lz77]

Why even if we have a hash table of 128K cells and remember hash for each position can often be done subj?
For example: let we found match from current position for substring 'abcd' in string ...zabcd..., then we found that 'zabcd' also matches.
Sorry for my English...

[Shelwien]

Fast encoding strategies like to skip hashing inside of matches.

Otherwise, you can just get collisions for hash values - its easily possible that the cell for 'zabc' would be overwritten, while the cell for 'abcd' won't.