Are you sure that your calculator shows correctly 24h dif? Take all values of these and divide by their quantity.
https://api.straks.info/v2/charts/difficulty?days=1

These 2 calculators do it in that way and have other (the same) difficulty comparing to your calc.
https://blockstats.pw/straks/
http://strakscalc.cba.pl/

Make sure you don't take always latest 1440 blocks (60 blocks per hour * 24h) but all existing values in above link above because block time fluctuates a bit and currently I see 1257 blocks not 1440.