Equivalence of correlated DATEADD intervals
In that case they are equivalent behaviors even though the ScalarOperator definitions are different. This is because in other cases they can't be swapped. Think about:


datediff in days, which depends on which month and also must account for leap year. 
datediff in seconds which will be non-deterministic when there is a leap second. 
etc. 


SQL Server doesn't gain any advantage changing one scalar operator's definition to look more like another one's. What would be the benefit? If you said WHERE column = 4 * 5 wouldn't it be confusing if the scalar operator said WHERE column = 2 * 10? Even if you have another query somewhere that uses 2 * 10?

In other words, if you said DATEADD(MONTH, n*12, ... I see no reason at all for SQL Server to change that expression to DATEADD(YEAR, n, ... just because we all happen to know that there are 12 months in a year. You asked for a delta by months, not years, so that's what the expression should show.