Setting intensity 20 will very likely result in lower valid shares, thus less pay.
You all probably already know the reason: the pool is calculating your Kh/s based on your VALID shares submitted.
And the lower Kh/s on pool is resulted from the invalid shares submitted. And the invalid shares are from a few sources. But there is only one them of you can actually improve by yourself.
Although you may see a higher Kh/s on cgminer by setting -I 20, your cards are working very very hard and sometimes results in bad calculations and therefore contributes a large portion of to your invalid shares.
Setting it to 19 will improves your valid shares and thus higher Kh/s on your pool at least by 3%, even though you are seeing a slightly lower Kh/s on cgminer.
For example, I have 4x 290x all are at -I 20 and getting 883Kh/s each based on cgminer(totaling 3530Kh/s), that is the actual speed of my cards are working at. However if I go to my pools, I will find different Kh/s sometimes higher (4000+) than cgminer and sometimes lower(3000), but mostly lower, because of the invalid shares are eating my cgminer Kh/s.
After setting to -I 19, my cgminer shows 871Kh/s reach so totaling 3484Kh/s (-I 18 will get me to 830Kh/s, which is too much lower), but now the pool is reporting even higher Kh/s than my cgminers(about 3600Kh/s) and 0% invalid shares almost every round.
Why higher Kh/s than cgminer? Because a lot of other people are using -I 20 because it looks nicer in cgminer, however getting invalid shares, and the pool is thinking YOU are using better cards because it only counts your valid share to calculate Kh/s.
Try it out yourself.
If you find this helps, you are more than welcome to tip
BTC: 15DARf9fpp8Hn7MvUW33StP3knsZgsa398
LTC: LYcc9VKguWKfrsP2T7y1S3BeL1c1MFbF2s
DOge: D66DinpDEE8mZuBYgBadC7By1yXeX656TZ
Coinye: 5ogyMLgRQmCWHq897o1btPXp2ibd7tGJYj
42: 4H3k5Msh31dJ9nEbKAeuYibQdaLS1JzVK5