Sometimes for example I change a word in line 100 and delete last 4 lines.
When I run "git diff" It shows a log which has a -(minus) and +(plus) before all lines whereas I changed only mentioned lines.
I thought it may be due to changing number of all lines but when I saw the changes again I didn't see any change to line numbers.
so what cause this?
That's just formatting to show what lines have been changed (added/removed/etc). Those +s and -s are not actually in your files.
Which text editor are you using? Perhaps it changed line endings between Unix/DOS. (i.e. LF to CR+LF)