You're right. I think it's just what people are used to. Compare:
"If two gallons is less than the amount of gas in your tank, you should fill it up."
"If you have less than two gallons of gas left in your tank, you should fill it up."
In everyday speech, as well in mathematics, it is usual to state the variable first (what are we're going to compare) and then the amount to which it will be compared.
The first is especially awkward because you forgot to flip the < into a >. It should be, "If two gallons is greater than the amount of gas in your tank, you should fill it up."
"If two gallons is less than the amount of gas in your tank, you should fill it up."
"If you have less than two gallons of gas left in your tank, you should fill it up."
In everyday speech, as well in mathematics, it is usual to state the variable first (what are we're going to compare) and then the amount to which it will be compared.