Strange Display Order of Title Tags in Easytag

Using Easytag 2.1.7 to edit tags of BBC Podcast mp3 files (In Our Time Archive).

These podcast files do not have a track number when downloaded but when viewed in Easytag they appear in alphabetical order of Title tag (possibly same as file name but not checked yet,) except for Titles starting with x,y or z which are put first and then followed by a,b,c etc.
For example “IOTS: Zero” is before “IOTS: Ageing,” “IOTH: Xenophon” is before “IOTH: Africa,” “IOTC: Yeats and Irish Politics” and two more IOTC: Y… are before “IOTC: Abelard…” Could somebody please explain why this might be?

I use Twonky on NAS device to distribute multimedia data and the absence of track number with these BBC podcast mp3 files also appears to cause strange results with uPnP client players and I think I shall need to use scanner to write track number based on alphabetical order but without the above anomaly. Any views on if this is the right way to proceed?

Grateful for advice/suggestions.