English Medical Medical (general) Cancer & mathematics_understanding tumor reduction

Hello again.
Can someone explain this summation/subtraction to me, please?

“This is to let you know that Mrs. XX is doing extremely well and she had more than 35% reduction in the volume of her disease compared to the previous time.
If we compare the volume of disease she had when she came here to the volume of disease that she has now, she is down now to 20% of the original tumor volume.”
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From the first sentence, I understand that the reduction is +35% and 65% of the tumor remains;

but from the second sentence I understand that the reduction is 80% and only 20% of the original tumor is remaining.

What am I misunderstanding?

Many thanks for your help,
Manuel