DOS 2.5 Enhanced Density Disk Detection

Can any DOS experts explain to me why DOS 2.5 would attempt to access sector 1024 (VTOC2) on a 720 sector disk? We're getting SIO errors following a directory read when 720 sector DOS disks are mounted via PBI as disk images, but not when they're running through an SIO device as normal. When listing a disk directory, DOS will read - say - two directory sectors, then make three (unsuccessful) attempts to read sector 1024, resulting in a NAK error which is reported on screen. This only happens when the disk is mounted under PBI control.