Say of the whole population GenZ is 25% and GenY is also 25% (for convenience). If then 28% of GenZ identify as LGBTQ+ that would be 28%*.25= 7% (25% of 28%, or 1/4 of 28%). For GenZ maths would be 16%*0.25=4%. So these two groups would in total contribute that 11% of the total population is LGBTQ+.
Numbers pulled out of my arse for illustration
Say of the whole population GenZ is 25% and GenY is also 25% (for convenience). If then 28% of GenZ identify as LGBTQ+ that would be 28%*.25= 7% (25% of 28%, or 1/4 of 28%). For GenZ maths would be 16%*0.25=4%. So these two groups would in total contribute that 11% of the total population is LGBTQ+.
Hopefully that makes sense.