Okay let's try this.

Values are changed for the sake of clarity.

Let's say a melee minion gives 100xp. When a solo laner is in exp range, they will gain 100xp. There is a separate value for sharing xp. Let's say shared xp is 70% of solo. So now in bot lane, when the adc and support are both in range of the melee minion, they will gain only 70xp each. This is the same if your jungler ganks mid lane and a melee minion dies while you're both in xp range.

These values are being changed this patch. Now melee minions will give 102xp solo, but the shared exp value has changed to 68.6% of solo xp. They are increasing minion xp, but decreasing the percentage of that xp you gain when more than 1 player is in xp range. So now solo laners are gaining 102xp per melee, but duo lanes are only gaining 68.6xp per melee.

I'm not sure if that helps. I think the way you're looking at it is that they are increasing minion xp, then decreasing the amount of xp duo lanes are getting from that. What you want to be looking at is current value which is what they're basing this on. Solo lanes are gaining 2% more xp than currently and duo is losing 2.4% more than currently.

If you and your friend had 10 apples, then they gave your friend 2 more apples and took about 2.4 apples from you, you would not say you only had a net loss of 0.4 apples because you weren't given the additional 2 in the first place.