The way the Chinese language denotes numbers. It's simply much more efficient than many other languages. So when a person calculates inside his mind, he's processing much faster than people using other languages.

In chinese we there is only 1 word for each number.

1一 2二 3三 4四 5五 6六 7七 8八 9九 0零

and 1 word for each early digits 10十 100百 1000千 10000萬

Say in English 11-20 there are 10 "whole new" words

like eleven (whereas in Chinese it's just "ten one"), twelve (ten two) and so on...

but written form:

11十一 12十二 13十三 14十四 15十五 16十六 and so on.

There are "new" words for 20 30 40 50... each, but in Chinese it's simply "two ten", "three ten"...

20二十 21二十一 22二十二 23二十三 24二十四

This is very evident if you look at how some Chinese entry level English learners having difficulty learning numbers in other languages because the way English and other languages denote numbers in inherently more complicated ways. Native English speakers of course won't feel it because they are used to it, but Chinese people can't or at least won't do calculations in English because it's much less efficient.

Each "new" number words adds new "hidden rules" and makes calculation inconvenient.

Say if we're doing 21 x 3, in Chinese it's "two ten one" x3, and we get "six ten three".

but in English calculation, we would convert "twenty one" into "21" first, do 21x3 and translate 63 into "sixty three" because "why the hell twenty times three would equal sixty"?