I was getting too many queued items so I decided to check the code that calculated these items, checking the queue.php (side note, maybe some more comments in the source code would help), I noticed line 1732:

$nextcheck = $delay * floor($now / $delay) + ($itemid % $delay);

I'm not 100% sure what this code does, but finding the $itemid here is quite strange, even more since the itemid value is different in case of distributed monitoring is a really large number and is different for each item, so I changed it to $now and the queue seems to be normal, although I'm not sure if I did the right change.