Average Time at a Pay Level, Slicing by Position Title

All - not sure if this is a Power Query or DAX issue, but:

I have a table of employees employee changes, including promotions. Like below:

EmployeeID    Employee     Old Pay    New Pay    PositionIDOld   PositionIDNew  Created Date    PositionTitleNew

1234               Smith, John      9            10                 452                    452                   1/15/2021          Developer

 2345              Doe, Jane        11          12                 341                     431                   1/14/2022          Human Resources

3456               Doe, John                      9                                             561                    6/12/2022         Security

What I am trying to calculate is the average time employees are at their Old Pay before being promoted. Blanks indicate the employee was a new hire. I have two columns which calculate the months and years an individual employee has been in the position - that part is working fine.

What I can't figure out is the averages. They don't seem to averaging correctly - I am confirming the results in Excel and via a calculator, but the results aren't matching up. I removed all rows in which someone's Old Pay and New Pay are the same (which could be because they moved jobs but stayed at the same pay, changed their name, etc.).

The table I created will only show averages up to a pay 10, even though I know employees went up to a pay 11. Simiarly, when I filter by position, there are Pays on there which aren't accurate - for example, it will say the average time a Developer is at a Pay 8 is 11 months, but Pay 8 isn't a valid pay for a developer. 

I tried changing the column to show the PositionTitleOld, but that didn't change the averages at all. Additionally, it left me with blank values because new hires don't have an old PositionTitleID. 

If it helps, this is the calculated column I'm using to calculate months at the Pay level - it is working fine. I have a similar one for years at the pay level.

MonthsinGrade =
var _min = MINX(FILTER(PositionHistory, PositionHistory[EmployeeID] = EARLIER(PositionHistory[EmployeeID]) && PositionHistory[CreatedDate] > EARLIER(PositionHistory[CreatedDate])), PositionHistory[CreatedDate])
var _diff = DATEDIFF(PositionHistory[CreatedDate],COALESCE(_min,TODAY()),MONTH)
return MOD(_diff,12)