Any Member Please Explain- I Don't Understand The Handicapping/Ratings.

Having just watched Willie Mullins’ interview on Racing Post- discussing his Cheltenham prospects- I was interested to hear his comments about the prospects of Scotsirish for The Glenfarclas Cross Country Chase at the forthcoming Festival.
If you remember, Scotsirish was challenging strongly round the final bend in the December version- when several of the runners took the wrong course.
From what I remembered, Scotsirish looked like winning, but Willie made the comment to imply that Emmet Mullins (riding Uncle Junior) was coming up just behind and going equally well, and “ he doesn’t get going until after 3 miles,” Willie added.
Having just watched the replay, it’s true that Uncle Junior came from nowhere and was finishing like a train – a couple of lengths behind Scotsirish- when they ‘ran out.’
Looking at the form of both horses, can a member explain to me their handicapping for The Grand National?
French import Uncle Junior only had two races after joining Mullins in 2008- and then never ran until the end of 2010.
After winning his first chase over 20 furlongs in March last year, he has had three races over extended distances- a win over 3m 6f (ridden off 130 by Ruby and reaching139 afterwards-normal chases); a second win over 3m 7f when he beat Garde Champetre at levels in last November’s Cross Country Chase; and his latest run against Scotsirish (at level weights) in December’s Cross Country fiasco.
For this race he was rated 156 against 153 for Scotsirish.
I notice that Garde Champetre had a rating of 159 when beaten by Uncle Junior- who had no rating but won at level weights.
Are ratings for Cross Country Chases different from other chases?In The Grand National, Scotsirish is weighted to carry 11st 2lbs and Uncle Junior has 10st 5lbs.
The main question is...
Why is Uncle Junior only carrying 10st 5lbs in The National- 11lbs less than Scotsirish?
Furthermore, given that Uncle Junior stays for ever and has never fallen, do members think he has a chance at Aintree- if he runs and gets in the race (needing 8 horses to defect)?

 

Bumping this up for Tam

 

Are ratings for Cross Country Chases different from other chases? - Yes, take a look at Double Dizzy and Wedger Pardy.

Why is Uncle Junior only carrying 10st 5lbs in The National- 11lbs less than Scotsirish? See above