The same round on an easier course

131 gives 17.6. 113 gives 20.4 — a difference of 2.8 more (+15.9%).

The same 92 is a worse differential from easier tees: the slope is the divisor, so a gentle course gives less credit for the same score.

The same round on an easier course, side by side
Input	131	113
Adjusted gross score	92	92
Course rating	71.6	71.6
Slope rating	131	113
Par	72	72
Your handicap index	14.2	14.2
Playing conditions adjustment	0	0
Handicap allowance(percent)	100	100
Score differential	17.6	20.4

Every figure, both ways

Every figure, both ways round
Step	131	113	Difference
Score differential for this round	17.6	20.4	+2.8
Course handicap from your index	16.06	13.8	-2.26
Strokes you receive	16	14	-2
Playing handicap after the allowance	16	14	-2
Net score	76	78	+2
Net score against par	4	6	+2
This round against your index	3.4	6.2	+2.8
Score that would match your index exactly	88.06	85.8	-2.26

Does lowering slope rating change the answer in proportion?: No, and that is the point of putting them side by side. Slope rating changes by 14% while score differential changes by 16%. The step table below shows which line breaks the proportion.
What is held constant between the two columns?: Everything except slope rating: adjusted gross score, course rating, par, your handicap index, playing conditions adjustment, handicap allowance. That is what makes the difference attributable — change two things at once and neither column tells you which one moved the answer.
Can I compare my own two numbers?: Yes, from the address bar: add ?a.slope=… and &b.slope=… to set each side, and a parameter without a prefix sets both at once. The page is computed per request, so any pair works — these two are only the ones worth a link.

Change either side by adding query parameters — ?a.slope=… and b.slope=…. A parameter without a prefix sets both sides at once. Source: USGA — Rules of Handicapping, World Handicap System — how the index is calculated