I have removed dumbbells underneath it that is why it has lowered a bit.
That is incorrect for several reasons. First, it is a run-on sentence. It should be two sentences. "I have removed some dumbbells ...That is why ...". Second, I don't think there is such a thing as a "decline bench." It should be "inclined bench." Third, it is more natural to say "I have removed some dumbbells from beneath it (or from underneath it)". Fourth, I think "has been lowered" is preferable to "has lowered."
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