Phonetic difference between Gloucester and Cirencester
The origin of the pronunciation of ancient towns' names is not always clear. Cirencester appears to be a sort of exception among other -cester names.
Cirencester:
In Anglo-Saxon times the name of the town was written Cirrenceastre or Cyrneceastre (the Saxon ‘c’ was pronounced like the ‘ch’ in change). The Normans mispronounced the ‘ch’ sound as [ts], resulting in the modern name Cirencester /ˈsaɪərənsɛstər/. The form Ciceter /ˈsɪsᵻtər/ was once used locally as an abbreviation. Sometimes the form Cicester /ˈsɪsᵻstər/ was heard instead. These forms are now very rarely used, while many local people abbreviate the name to Ciren /ˈsaɪərən/.
Today it is usually /ˈsaɪrənsɛstər/, as it is spelt, although occasionally it is /ˈsɪsᵻstər/, /ˈsɪsᵻtər/ or /ˈsɪstər/.
(Wikipedia)
-cester:
Why is “-cester” given a clipped pronunciation in place names? The OED says only that “the history of the form written -cester, of which only -ster is pronounced (in Worcester, Bicester, etc.), is obscure.”
It’s difficult to trace the pronunciations of place names, since we have only written records to go by, and many old pronouncing dictionaries don’t include place names.
Two that do, however, might lead us to believe that the “-ster” pronunciation developed in the early 18th century.
Thomas Dyche, in A Guide to the English Tongue (1709), gives three-syllable pronunciations for the three cities, which he renders as “Wor-ce-ster,” “Lei-ce-ster,” and “Glou-ce-ster.”
Half a century later, William Johnston’s A Pronouncing and Spelling Dictionary (1764), in a table devoted to “Words With Quiescent Consonants,” says the “c” is not pronounced in “Worcester,” “Leicester,” and “Gloucester.” (This makes them two-syllable words.)
So it would seem at first glance that the “-ster” pronunciation established itself sometime between 1709 and 1764, assuming these lexicographers were in touch with local usage.
(Grammarphobia)