Doing chomod 755 on a directory gives me drwxr-xr-x. Should it not give me drwxr-xr-xr since 5 is read,execute?
And Doing chmod 751 on a directory give me drwxr-x--x. Should that not give me drwxr-xr-x?
The bits are groups of three the results are correct, I do not really get the point you misunderstood. 5 is read, execute but the order of rwx does not change. Consider them as switches, they might be on (r/w/x) or off (-). Here again an overview of the values: