Some polyurethane finishes come in different sheens: gloss, semi-gloss, satin, flat, etc. They start out glossy, but add flattening agent to the finish to achieve the desired effect.
Sometimes the flattening agent can settle to the bottom, so you end up with a glossy finish on top, and a flatter than expected finish on the bottom. The trick is to mix the polyurethane very thoroughly before you use it, to make sure the flattening agent is well-blended throughout. Be patient and stir very slowly, to avoid introducing bubbles.
As mm_wood pointed out, the sheen can vary between batches. If it is too glossy, then you may be able to fix it by applying it to the rest of the floor to even the coat, go back and gently sand with a fine grit (say 220 or 320), and then reapplying another coat of flatter finish on top of it to even it out. If it is too flat, then applying another coat of glossier might help, but the flatter finish is still underneath. Yeah, sanding it off sucks.
You are spending a lot of time and effort, so take extra time to test on scrapwood, and then in an unobtrusive corner first.