Why the converter may return several lines
The range 192.168.10.10 through 192.168.10.25 is covered by 192.168.10.10/31, 192.168.10.12/30, 192.168.10.16/29, and 192.168.10.24/31. Together these blocks include exactly 16 addresses. Rounding to a convenient /27 would include additional addresses.
This is address-set arithmetic. It does not determine whether addresses are routable, whether hosts exist, or whether a firewall rule is suitable for your service. Review the resulting policy in your own equipment before applying it.