Idk what it means.why is it not reasonable to say that 4.23 is less than 4.135

it is unreasonable because 4.23 is bigger than 4.135! Look at the 10s place - 1 is smaller than 2, and everything afterwards is irrelevant. There could be as many zeros or any combination of numbers you want, but as long as that tens place is larger (2 vs 1), than it's not reasonable to say 4.2 is smaller!!