why 1 month data file is named as 43200, not 40320?

 
MetaTrader 4 data files is under 'history' directory, such as

usdcad1.hst is 1 minute chart
usdcad5.hst is 5 minute chart
usdcad15.hst is 15minute chart
usdcad60.hst is 1hour chart
usdcad240.hst is 4hour chart
usdcad1440.hst is 1day chart
usdcad10080.hst is 1week chart
usdcad43200.hst is 1month chart

The number behind 'usdcad' seems like the number of minutes according to the chart, just as 1 hour =60 minute, 1 Day =240 minute, 1 week =10080 minute. But if we calculate like this way, 1 month should be 40320 minute. So why 'usdcad43200.hst' is 1 month chart, not 'usdcad40320.hst'? Any reason behind that?
 
1 day contains 1440 minutes always
1 week contains 10080 minutes always
1 month? from 40320 to 44640 ie floating value. 43200 = 1440 * 30
Reason: