First off, I would note that your ancestor initially served in Company E, 4 Regiment Six Month Militia. He mustered in at Perryville on Oct. 21, 1861, and mustered out four months later. Then on April 19, 1862, he enlisted in Company D, 12th MSM Cavalry, which was commanded by Capt. William Flentge. These facts are highly relevant in regard to answering your initial question.

To see a detailed overview regarding what was going on in and around Cape G during the immediate timeframe surrounding your ancestor's enlistment in the 12th MSM Cavalry, read the story found here -- http://www.civilwarstlouis.com/JacksonFairgrounds/index.htm It has ten parts, including the introduction, and the epilogue (all of which are also highly relevant to your query).

Anyway, in the story note the discussion regarding the pressure area Unionists were under from opposing forces that resulted in them enlisting, note the names of the commanding officers who play primary roles in the story (especially Murdoch and Flentge), note the dates involved, and note that in the immediate aftermath of this affair troops from Perry County came down to support the mop-up operation.

I suspect that your ancestor came down as part of that mop up, and a few days later he ended up enlisting in Flentge's company, which had been depleted of manpower as a result of the events outlined in the story....

Anyway, should you have any additional questions, please let us know.