Convert "Spaces to Tabs" doesn't work if (no. of spaces % tab_width) != 0

Hello!

        12345678901234567890
       12345678901234567890
           12345678901234567890

For line 1, there are 8 spaces at the beginning and so convert "Spaces to Tabs" converts those 8 spaces (tab_width) to 1 TAB. For line 2, there are 7 spaces at the beginning and so convert "Spaces to Tabs" should have converted those 7 spaces to 1 TAB. But it leaves those 7 spaces as it is. For line 3, there are 11 spaces at the beginning and so convert "Spaces to Tabs" should have converted those 11 spaces to 2 TAB chars. But it coverts first 8 spaces to 1 TAB and leaves remaining 3 spaces as it is.

Also, "Spaces to Tabs" should be renamed to "Leading Spaces to Tabs" as unlike "Tabs to Spaces" it converts only the leading spaces. (I wasted a good time confused by this)