I think this is simply because before a certain time place names hadn't yet been formalised.

As an example here's an extract from a memorial from the Registry of Deeds I transcribed a while ago where the various spellings of townlands are given. My transcription is incomplete not just because the document was hard to read but also because I wasn't able to find other references to the earlier variations of the place names involved.

Quote:
The castle, mansion house, town and lands of Foxhall, otherwise Ratherlough otherwise Rathreagh, Corsebullaghs otherwise
Com_ebullaghs otherwise Commebullaghs or Corabola Rossart, Aughadrussagh otherwise Carrick__amill, Aghaneskeoge otherwise
Anoghanliskoge, Claghamore otherwise Claghamore otherwise Cloghnamore or Cloghamore, Aghnakelly otherwise Aglinakelly otherwise
Aghanakelly, Aghanregby otherwise Aghnemecloge, Aghande__y, Cloghame, Rath, Clondamogh and Carduff, the towns and lands of
Cloghiggin otherwise Cloghen otherwise Oghaneaslau otherwise Aughorslane otherwise Sillinreagh otherwise Skillianreagh otherwise Clygeen
otherwise Cloghger and Smithfield, the town and lands of Aghavanagh otherwise Portmahon otherwise Newport, Aghnasiloge, Newtown,
Carigin otherwise Carrageen, Killeencrobagh otherwise Killen, Kinnard or Killeen and Kinnard Largie otherwise Lurgee otherwise Lurgan,
Tonebegg otherwise Sca__an Slyaun otherwise S___anmogg, Clonferrock, Slyan Sankey, Furze and the rectorial tithes of the parish of
Foxhall otherwise Rathreagh all situate, lying and being in the Barony of Ardagh and County of Longford