question about sameperiodlastyear

This is my original form  sheet1

date             quantity

2017/1/1         1

2017/1/2         1

2017/1/3         1

2017/3/25       1

2018/3/5         1

then i use calendar(FIRSTDATE('Sheet1)'[date]),LASTDATE('(Sheet1)'[date]))) to create a table, it has dates from 2017/1/1 to 2018/3/5

create a function 

last year=CALCULATE(SUM('Sheet1'[quantity]),SAMEPERIODLASTYEAR('表'[Date]))

for 2018,why it shows 4?

the max day is 2018/3/5,Its previous year's date is 2017/3/5 ,sameperiodlastyear should return dates before,it should return 2017/1/1,2017/1/2,2017/1/3

why the result is 4?not 3?