Nice question as many people find this confusing, you can understand this way as lead acid battery manufacturer designs batteries in 12 Volt format. So UPS designers either design a system either in format to get connected with 12 volts lead acid battery or with two lead acid battery connected in series to make it a 24Volts system.
Connecting batteries in series increases voltage but does not increases AMP-HR ratings, but why would a customer be concerned that what is going inside as you want to see things in terms advantage you will get, so keep reading on , that just says that charging rate for 12Volts battery bank with 160 amp-hr rating battery will be similar for 24 volts battery bank system with two 160 amp-hr ratings batteries connected in series, which in our case is 160/10 is 16 amperes of maximum allowable charging rate for both 12volt system and 24volt system. But obviously, backup time for 24 volts systems will be double then that of 12 Volts system.
Voltage adds on in series whereas current remains same, that?s why charging rate for both the system would remain same, if you want to consider safety and don?t want to destroy your battery bank earlier.