When a number has a single digit (number) in the "thousands"
group and when the last two digits are 00, people usually say
the number as "___ hundred."

Examples:

Native English speakers do not say large numbers this way
if the last three digits are 000:

1,000 = one / a thousand

but 1,100 = eleven hundred (or one thousand one hundred)

6,000 = six thousand

but 6,500 = sixty-five hundred (or six thousand five hundred)

1,900 = nineteen hundred (or one thousand nine hundred)

but 2,000 = two thousand

3,900 = thirty-nine hundred (or three thousand nine hundred)

but 4,000 = four thousand

__________________________________________________

Special Note:

Some people say numbers less than 10,000 this way even if
the last two digits are not 00:

2,125 ---> twenty-one hundred twenty-five;

5,234 ---> fifty-two hundred thirty-four;

9,999 ---> ninety-nine hundred ninety-nine.

Saying large numbers in this is not very common, however.