How to interpret increases in percentage with different sample groups
If you ask me your question is better suited to cross validated (stats.stackexchange.com). 

Anyway you're probably confused because the percentages refer to different base lines. Note that the 60% of disease at B in 2017 refers to 40% of the population, whereas the 30% in 1917 refers to 30% of the population, so they are hard to compare.

Try working it out in absolute numbers to get a clearer picture:

In 1917 if there are 100 people, 70 of ethnicity A, 30 of B. Of the 70 people of A 7 (10% of 70) get sick, of B 9 get sick (30% of 30)

In 2017 there are 100 people, 60 of ethnicity A, 40 of B. Of the 60 people of A now 18 catch the disease (30% of 60), and of B 24 catch the disease (60% of 40).

Observations: overall sickness has increased (16 vs 42). The absolute increase in sick B people (15) is larger than that in A people (11), but this in part due to the fact that there are more B people now