The terms are not always completely accurate so use your best judgement. I've often seen large onions that were as big or bigger than "jumbo" according to this data.
There is one reason I might do this. If the core is green, that is starting to form a sprout. I cut that out as it can be bitter. Otherwise I don't know why one would do that.
Some slime building is normal in onions and related vegetables, see this question. It is less pronounced in bulbs than in leaves, but it can occur. If your onion bulb is solid, there is no problem with it.
Sometimes you can have spoiled slimy onion bulbs. In this case, spoilage microorganisms eat away the firm cell structure of the onion and leave you with slimy mush. You can recognize this easily: if it has happened, the onion has a patch which feels really soft, and is much slimier than the rest. It is often discolored, and can also smell strange. It looks and feels very different from a healthy onion with a slight slime layer. I can't be 100% sure as I haven't seen your onion, but from your description, it sounds like it wasn't the case this time.
Best Answer
Here's some general guidelines I have used in the past:
source
The terms are not always completely accurate so use your best judgement. I've often seen large onions that were as big or bigger than "jumbo" according to this data.