I would trust that your slicer is more correct than Blender, probably something is off with how you are measuring in Blender as it doesn’t appear to be a scaling issue since one dimension is larger and the other smaller. I’m not too familiar with how Blender works below the hood, but I am pretty sure it isn’t off by even 1%, but there is a good reason why CAD software kernels exist that use parametric equations (e.g. NURBS) to precisely represent dimension critical designs.

I don’t think this is the case, but one potential issue to be aware of is that STL files are really just a faceted approximation of the true 3D data, depending on how it is exported the facets can introduce dimensional inaccuracy. So many people export their models with hilariously poor resolution that could result in dimensional discrepancies. But that is only the orders of a couple tenths of mm maximum, not what you are seeing here.