In the standard implementation of LaTeX2e there is deficiency around handling in-text floats which indeed results in not applying the space equally. In the answer to the question "intextsep gives doubled space" I discuss this in some detail including ways to improve that situation.
There is a related issue in that even if the spacing above and below is made equal there is the issue that you get adjustments due to the depth of the line in front of the float and those adjustements then make the float visually go off-center. How to improve that is discussed in "parskip inserts extra space after floats (and listings)".
Finally the content of your float may have extra space at the bottom or top if you have a caption there as the caption has a
\abovecaptionskip (usually positive and a problem if the caption is on top of the float or table) and a
\belowcaptionskip (usually zero, but may not be and then a problem if the caption is below the float.