If you run this code :
a = 1640799.87
print( "{:,.1f}".format(a) )
on the client is will show (incorrectly) :
1640799.9
and on the server it will show (correctly) :
1,640,799.9
Demo here :
https://anvil.works/build#clone:5DGI35EEPXAGXJ2Q=5ZY636RC6GDKWQA42HE3E37J
Click the button on the form and the same code will run client & server side and print their results.