What is the result of each individual statement?

SUM(Case When ah.EntryDate Between '12/01/2020' AND '12/02/2020' Then 1 Else 0 End)

SUM(Case When ah.EntryDate Between '12/01/2019' AND '12/02/2019' Then 1 Else 0 End)

Are you trying to find the percentage of entries this year from the total? If so - wouldn't you need this statement after the division?

SUM(Case When ah.EntryDate Between '12/01/2019' AND '12/02/2029' Then 1 Else 0 End) + SUM(Case When ah.EntryDate Between '12/01/2020' AND '12/02/2020' Then 1 Else 0 End)

And don't forget about integer division...if you have, for example a total of 5from the first part and divide that by 10 for the second part (e.g. 5 / 10) the result will be 0 and not 0.5. To get to 0.5 in that situation you need to change one of the parts to a numeric value:

SUM(Case When ah.EntryDate Between '12/01/2020' AND '12/02/2020' Then 1 Else 0 End) * 1.0