Why do some of my portfolio items have a grey line?

In some of my portfolio items, on the main grid, some items have a grey line showing and others do not. It looks like it’s the border but moved up. And at first I thought it was the length of the title because for the first couple of rows, items with a long title that went onto a second line didn’t have it, and short titles did. However scrolling down further I found several single line titled items with no grey line. All the images appear to be the same size.

So now I’m stumped. What are we doing that results in some items showing a grey line and some do not. We don’t mind if it shows or it doesn’t. We just want them all to be the same.

Thanks in advance for any insights you can offer.