Mine are the same, esp Jaffa. He always uses a corner or side. Fortunately they use a covered tray so it's ok. I think with Jaffa it's because he wants to mark the tray with his scent and weeing against the side is the next best thing to out and out spraying.
Perhaps if yours don't like covered trays a high sided one like the one below might help prevent overspill!
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