Eh? Well, the way I was taught to calculate a percentage increase was:
(a) 26.3 - 25.6 = 0.7 (b) 0.7/25.6 = 0.02734
It's not a percentage increase in the first place. When the power of the incident light was 100W the old solar cells would produce 25.6W of electrical power. In the same conditions the new cells provide 26.3W. So that's an increase in the delivered power of (26.3 - 25.6) / 25.6 = 2.7%. See none of the numbers being compared are percentages.