All Republicans and all Democrats don’t act and think as one. Take any collection of humans, of any size, and their opinions will be spread across a spectrum.
There are, and always will be, some members in either house and of any party that find it’s in their political interest to debate and pull new laws left or right.
This is especially true under FPTP because each member has to answer to an area, and different areas of a country have baked-in differences in their ideal political outcomes and points of view.