You can report issues here: https://github.com/mendix/CKEditorForMendix/issues
It says "500 characters, including HTML". The CKEditor is a WYSIWYG editor that will create HTML, so there are hidden tags, which adds up to 500 characters. You could create a new text-area in your test-project, point it to the same attribute and see what the widget will store as text in this attribute.
Secondly, I would suggest you set the limit of the attribute a little higher than what you set for the CKEditor. This way you will always make sure it doesn't go over the limit