Each bin defines the absolute maximum number the bin can contain. A bin of 10 will contain numbers up to 10 so 9 and 10 but not 10.1. In other words, the bin contents are less than or equal to (<=) the current bin and greater than (>) the previous bin.
In short a bin contains numbers up to and including the bin’s number but not a fraction over the bin’s number as shown in the examples above.
People usually give their age as rounded down and age limits operate in that way too so a 17.9 year old can’t vote in the UK. If we want to find the number of people aged say 15-18 we need to use the Excel ROUNDDOWN function to make sure someone who is 18.3 or 18.9 appears in a 15-18 bin defined as 18. The previous bin in this example would have to be 14 to capture 14 year olds but not 15 year olds.