Due to Uthman's systematic destruction of the early Quranic manuscripts, very little evidence remains of what the great pre-Uthmanic Quran teachers taught. Much of what we know has been saved through quotations and traditions faithfully recorded by early Muslim authors such as Ibn Abi Daud (none other than the son of Abu Daud, compiler of one of the collections of sahih sittah). As fortunate as we are to have these records of early variants, very little can replace the power and poignancy of manuscript evidence.
As fate would have it, however, a palimpsest was discovered which contained a few notable similarities to the supposed Ibn Masud manuscripts. (A palimpsest is a manuscript which has been washed out and written over; the original text can still be examined, however, via UV light imaging methods). Within this manuscript are no less than seven types of variances from the standard texts of today:
1 - Different sequences of words
2 - Omissions
3 - Different words
4 - Orthographical variants
5 - Scribal errors/corrections
6 - Explanatory additions
7 - Different contexts which are a result of omitted words.
Below is an image of Fogg's Palimpsest. Notice that the scripto secunda, or the overlying text of the palimpsest, was made to match the standard text; the scripto prima, or the older text, had different wording.
Why do Muslims continue to deny the existence of Quranic variants from early Islamic history? Well, it's because they must if the Quran's claim in 15:9 is to be defended. But just how long can Muslims continue to deny the existence of variants while staring at variants? Only time will tell.