dateDiff odd count of last day inclusive/exclusive

Hi, I’m confused about the count of weekdays between two dates with dateDiff.

If the starting date falls on a weekend (Saturday or Sunday) the count becomes inclusive of the last day.
If the starting date falls on a weekday (Monday to Friday) the count becomes exclusive of the last day.

I would expect it to be always either inclusive or exclusive, but not both?

To compare I used the calculator from this site Business Days Calculator – Count Workdays with the option to exclude weekends.

For now I do it like this, to get the desired output:

function getWeekdayCount(start, end, inclusiveLastDay = false) {
    var diff = dateDiff(
        datePart = server.coldfusion.productname EQ 'Lucee' ? 'wd' : 'w',
        date1 = arguments.start,
        date2 = arguments.end
    var isWeekend = listFind('1,7', dayOfWeek(arguments.start)) ? true : false;
    if(arguments.inclusiveLastDay) {
        if(!isWeekend) {
            diff += 1;
    } else {
        if(isWeekend) {
            diff -= 1;
    return diff;