I abstracted the Kentucky Confederate Pension files and published the information in "Confederate Pensioners of Kentucky" in 2000.
In my experience with the applications, few of the widows actually knew the unit in which their husbands had served. When it came time to apply for a pension, I believe that they probably contacted another Confederate veteran in the community and were eventually put into contact with veterans that served with the deceased husband and could testify as to his service. Keep in mind that the pension law did not go into effect until 1912. With that passage of time most of these veterans and widows were more concerned with qualifying for the pension and probably did not consider that their descendants would someday review these documents (My G-G-G-Grandfather probably would not have described his "piles" or hemorrhoids quite so well!).
In my book, from the information in her pension file, I have that John C. Lewis served in Company B, 5th Kentucky Infantry (1st Organization), rather than the 5th KY Mounted Infantry. You probably already know, but there is a big difference between the two units. Unfortunately, all of the records of the 5th Kentucky Infantry organizations have been lumped into one set of microfilm entitled "5th Kentucky Mounted Infantry" and this causes much confusion. The 5th Kentucky Infantry (1st Organization) was a one year unit. All that Lousinda (Ingram) Lewis had to prove to obtain a pension was that her husband was honorably discharged from his one-year of service in this unit. It is possible that she may have been avoiding mentioning one of these units, but I doubt it. I believe that she probably put down just what was necessary to obtain the pension and did not know about or care to elaborate on other enlistments. This might particularly be the case if John deserted from any of the other units.
Incidentally, I published "Confederate Soldiers of Kentucky" last year and also have the listing for Lewis in Fields' and the 7th Battalion Confederate Cavalry. I believe that you are correct that this is the same man. Have you seen Jeff Weaver and Jim Prichard's 7th Battalion roster which is posted online?
I hope this helps!