SQL Server - Why do I end up with 100.2%?

You ARE doing rounding, whether you think you are or not...your percentages aren't coming out exactly to 1 decimal place, so the values is being rounded to that degree of precision. Consider that each individual value could be off by 0.05% either way...that means the total for 8 such values could vary by as much as 0.4%. If you want more precision in the total, take the individual values to more decimal places.