What is traditionally called spelling pronunciation is actually a misnomer: it should be called reading pronunciation because all such incorrect pronunciations actually arise in the process of reading unfamiliar words rather than spelling them. Modern dictionaries such as The American Heritage Dictionary of the English Language (4th ed., 2006) typically dignify these errors by listing them alongside the traditionally correct form. For example, the reading pronunciation of equinox, with the same first vowel as echo, is given (second) after the form with the same first vowel as equal.
For the most part, reading pronunciations arise in words of Latinate (Anglo-Norman) origin, specifically and primarily as concerns the vowels of a given word. Here, British English has a long and venerable tradition of Anglicizing the pronunciation by rendering the vowels as diphthongs. Hence, for instance, instead of pronouncing pace ‘with the permission of; with deference to’ (< Latin pāce, ablative of pāx ‘peace’) as [pɛ́isi:] to rhyme with racy, speakers who have never actually heard this word uttered by a knowledgeable person will pronounce it [pɑ́čɛi], i.e. the stressed first vowel to rhyme with pocket, the unstressed second with hay, in accordance with the misguided American practice that makes Latin into a kind of Italian, and in fact it is this pronunciation of pace that is registered in The American Heritage Dictionary, which lists it first.
It is ignorance of the traditional anglicized pronunciation––nothing more, nothing less––that accounts not only for the erroneous pronunciation of Latinate vocabulary but of foreign nomina propria like Ossetia/Ossetian and Iran/Iranian. In each such instance, the traditional diphthong of the stressed vowel is replaced by a monophthong that is the result of a reading pronunciation.