What did I do wrong, finishing oak with spar urethane.

I just finished a small outside oak (pretty sure) deck with Minwax Indoor/Outdoor Helmsman Spar Urethane and was upset with the results. Some of the wood darkened considerabley while other places the finish came out nice and light.
The deck is outside but does have a small roof that covers it from a vertical rain. It was finished several years ago with the same finish. It had weather badly and where there was foot traffic the finish was missing and in places the wood was darkened. I used a small sander to get all the wood looking like "new". The deck was swept and wiped down with a towel until very little dust came up.
I did a Google search and found that polyurethane will darken the wood but what puzzles me is that some of the wood strips the finish looked good and other sections darkened way too much.
Did the urethane finish "lift" up something that was dark and in the wood just beneath the surface.
I read in the Google search that someone suggested using a water based finish.
Any help would be appreciated,
Learning the hard way %^(
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Oak has Tannins in it that make it darken if exposed to certain chemicals. That may be part of the problem. some kind of sealer might have helped because of the open grain.
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Young Carpenter

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