The numerical arrangement of the wonders is a conflation of the 20 wonders found in the fullest lists of the Irish versions of the Historia (See Translations: though note that this list is taken from a collation of manuscripts not a specific manuscript), with the 20 overlapping wonders from the lists of the British versions.
The following table makes explicit the derivation of the ordering. It is notable that there are 20 wonders in each list:
|Number||Wonder||British lists||Irish lists|
|3||The Fiery Pool||3||3|
|4||The Salt Fountains||4||4|
|5||Two Severn Kings||5||5|
|7||Fount Guur Helic||7||6|
|8||The Appled Ash||8||7|
|9||The Wind Hole||9||8|
|10||The Levitating Altar||10||9|
|11||The Returning Plank||11*|
|14||Cruc Mawr Tomb||14*|
|15||Brebic's Stone Cataract||12|
|17||The Well of Bones||14|
|18||The Undersea Birds||15|
|19||The Limpets of Ceoil||16|
|20||The Screams of Glen Ailbe||17|
|21||The Sealess Shore||15*||18|
|22||The Circling Rock||16*|
|23||The Swelling Shallows||17*||19|
|24||The Walking Stone||18*||20|
*Missing from Manuscript D, though whether this is true of all Gildasian manuscripts (See Manuscripts) is not obvious.
The British lists are likely to be the earlier ones, as in the Irish versions the text has been edited down considerably. However, the insertion of Irish list numbers 12-17 between the elements found contiguously in the British lists might suggest a fuller initial list of 26, at least for the Irish versions.
British list numbers 1-14, albeit rather disparate, appear to be associated together in a single list in the manuscripts, whereas British list numbers 15-18 are explicitly listed as new list, enumerated from scratch (the connection between these and British list 19 and 20 is ambiguous, but 15-18 are consistently numbered off in the manuscripts, whereas 19 and 20 are not, suggesting a new list).
British list 1-14 cover a wide geographical area centring on the "Old North" of the Scottish borders, Gwent, and the English borderlands. This may suggest collation during a very particular period of British retreat, perhaps re-authored after the lost of the English borderlands to the Saxons (some of the wonders are mentioned as being in the lands of the Saxons).
British list 15-18 are explicitly connected with one region (Mona) and this geographical focus appears to be the case with the Irish lists. This may suggest, along with their position in the manuscripts, that 1-14 represent the oldest core of the lists, and that this core has accumulated newer region-centred lists to it later. However, there is plainly both variation in style in the core list, and a geographical limitation within it to the areas above.
Strangely, the two Irish wonders (British list 19 and 20) don't appear in the Irish manuscripts, though Loch n-Echach does appear in an Irish list associated with the Irish Historia manuscripts: "The Wonders of Eri according to the Book of Glen-Da-Locha" (online).