leveling/ experience questions

i have some questions about in game experience and leveling.

i was playing top recently as shyvana, and i was crushing my opponent (shen) in laning phase. i had about double his cs at 15 min ( around 125 ish) compared to his 50 something. I was 2-1-0 of which both kills were shut downs from the enemy jungler. shen was 0-0-2. i believe they had a dragon at this point.

however, despite the gold advantage i had over him, he was still hitting levels before i was. he was 13, i was 12.

my question consists of a few parts.

1. how can i have such a huge gold/ cs lead and still be behind in levels.
2. are assists worth a large amount of exp? ( about how many cs worth?)
3. is dragon worth a sizeable chunk of exp? i know its hotly contested since season 5, but im curious as to how this can happen.

i guess what im asking, is how do experience and leveling mechanics work, as from the numbers ive posted, it shouldnt have even been close in levels. i was crushing him anytime we started a duel, yet he still had an experience advantage over me.

your responses are greatly awaited.