XP & Gold rewards are guaranteed and raised by each star you get... but its a lil bit different on the item side, these items are NOT a guaranteed reward, these are possible rewards and its, more or less, luckbased:
White means its a common drop:
- base drop chance without bonus is 25%
- mastery (a star) increases the dropchance by another 5%, so you have 40% with three stars
- items with + item increases the dropchance by another 2.5% for S, 5% for M, and 10% for L
- finaly there is a cap on 70%
So you have a chance between 25% and 70% that the white item drops if you win.
Red means its a rare drop and here the dopchances are a "lil" bit different^^:
- base drop chance without bonus is by 5%
- mastery increase the chance by 1% for a star... but sadly there is a bug, only the first star counts, so your chance is only increased by 1%, equal if you have one, two or three stars, so you have 6% with star(s)
- items with + item increases the dropchance by 0.5% for S, 1% for M and 2% for L
- and finaly there is a cap at... not 100% sure here, normaly is 12%, but because of the bug its maybe lower
So you have a chance between 5% and 10 (or 11... or 12. cuz of the bug...)% that the red item drops if you win.
So you dont miss something, but you may need a better dropchance, and this can be done with some + item, like "thief ring".