Hiya!
I wasn't trying to be condescending. I was trying to convey the general feeling of "don't expect every little thing to be able to be broken down into numbers and specific-word-designations as 3.x/PF/4e does". The way those games are written, the expect that the reader will "need" to know how every attack, every point of hp/ac/attack/etc, every spell or ability, etc. "fits" into the game rules. In those editions, simply saying "There is a magical shield protecting the amulet from removal" is, game mechanically speaking, simply impossible without opening a whole can of worms. The magic shield would/should have to be codified into specifics; does it have an AC? does it use the Hardness/HP factor? What is the DC to dispel it? Does it fall under Spell, Supernatural Ability, or some other "word designation". That is important in those editions because everything hinges around knowing that stuff (e.g., maybe a PC has an ability that allows him Spell Resistance to all but Supernatural Abilities...in which case "Magic" or "Supernatural Ability" makes all the difference in the world). With 5e, you can't "think like that", because the 5e rules are designed around a fast-n'-loose play style where not everything needs to fit neatly into some particular box. If a player says "But my character has X ability, can I bypass it?", then the DM steps in and does his DM'ing job of adjudicating the situation and makes a ruling ("Rulings, not rules" is frequently tossed about when talking about running/playing in a 5e game). Sorry if my initial post came across as condescending.
Looking at the Cat, as per your example, would you rule that a house can can carry 45 lb, let alone have a 90 lb person stand on it's back? Of course not...but "as per the rules", it can with it's STR 3. So, if you are going to "adjust those weights down to realistic levels" (re: make it up), then making the assumption that a Cat gets +0 to its Claw attack seems likewise "adjusted to realistic levels" (re: it was made up). That's how 5e rolls...

I'd suggest a 1 minute to 3 minute "look up" table rule; if you can't find specifics or a rule that fit's a situation...then make it up. That seems to be the go-to method for 5e, as I've said many times.
^_^
Paul L. Ming