SAMEPERIODLASTYEAR(DATESYTD( )) different from DATESYTD(SAMEPERIODLASTYEAR( ))??

This is just another case where the Time Intelligence functions don't work as expected because the Date table is wrong.

Remember: Time Intelligence functions are guaranteed to work *only* if the Date table is "complete" - meaning you need all the dates until the end of the year.

Not doing that, what happens is that the year 2019 seems made by 15 days, so if you include all of them than it seems you have an entire month, or an entire year.... You are violating the assumption made by Time Intelligence and you cannot trust any result at that point.