The story isn't about Zoro and I'm glad it is the way it is. The world building is far more interesting than the fights and Odas powerscaling was never logical to begin with. One piece is not a fight manga, it's an adventure manga.
Zoro is just gonna get whatever boost is necessary to deal with the obstacles of the plot and if Oda focused too much on the power level, the overall story would lose focus and start to suck, not to mention it would probably be far too long. Keep Zoro's fights very short and take up little space.