Q about diff

[KAWAI,TETSUYA-RC (HP-Japan,ex6)]

Hello GNU users,

I have a question about diff command.
I look io.c source code and think that the result of diff may be wrong
in the case that HASH value is same in some different line in a file.

For example, there are 2 file.
File a includes 2 lines and both lines have the same HASH value (i.e.
84736292).
File b includes 1 line and it has the same HASH value.

# more a
xxxxx => 84736292
yyyyy => 84736292

# more b
yyyyy => 84736292

In this case,  line yyyyy in both files are thought as different line by
diff command.

I think it is very rare case, but possible.
Is it correct ? If it's correct, is it correct behavior ?
I need confirmation from expert. 

Best regards,
- Tetsuya Kawai