One thing I don't understand is when I get more RP after winning and sometimes the RP I get is very less.

The RP earning is affected by 1) the cards that you use and 2) the amount of your staked SPS. You get bonus for using gold foils, alphas, betas and promos. On the other hand you get reduction if you use starter (not owned) or low lvl cards. At the battle results hover over the information next to RP earned (the i in circle) and it will give you breakdown of bonuses and reductions.

And welcome back to battling!