Renée K. Carl
This is due to Ancestry.com (as well as the Ancestry-owned fold3) taking license with the title of their digital index. The source for these cards is actually titled by NARA as
General Index to Pension Files, 1861-1934. Washington, D.C.: National Archives and Records Administration. T288, 546 rolls.
This includes service by some Civil War vets in earlier wars, as well as pensions granted for later events such as the Spanish-American War.
Note that there is another version of this index, T289, which is on Fold3. There are very similar, but not identical, and one can sometimes provide essential information for the locating the pension that the other cannot. Like Ancestry.com, Fold3 mis-titles the digital index as only Civil War, when in fact it is "Pension applications for service in the US Army between 1861 and 1900"
If the veteran in question died by 1934, the pension file is at the National Archives facility on Pennsylvania Ave in DC. If a widow or dependents lived longer, the file might still be in storage in Missouri, or at the NPRC in Missouri. It's been a while since I've worked on pension files, I am not up to date on what has transferred.
Hope this answers your question.