Because your solvent absorbs UV to some extent, too.
If you constantly see a negative dip, it may come from your sample, diluent, mobile phase, bottle, cap, glassware, column problems etc, that some compound is eluting out but does not absorb UV to the extent that your solvent does.
If there is a difference in composition between the injection solvent and mobile phase, your diluent (eg. water) could have a lower optical density than your mobile phase (eg. methanol) does. The negative dip will appear right before your sample is eluted.
*LC grade means that UV light absorbing impurities have been removed
(not that the absolute purity is higher) and absorbance of specified
wavelengths is kept within a specified range.
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