When applying the general Baseline Grid for the whole document and have a bigger space defined in pt between paragraphs than the font size itself, depending on how big the space is, the drop caps are not aligned to the top but a line below. In some cases it's ok, in some not. Maybe it´s also a combination of space before and after a paragraph.
Font: Akrobat (regular), Text height: 10pt, Paragraph leading (default 10pt), Space after paragraph 12pt. Se further screenshots which show different spaces between paragraphs.
Further finding: Text Frame/Vertical Alignment/Baseline Grid overrides the general document Baseline Grid and seems to work fine according to drop cap aligning.