Hey - the important thing to remember here is that you are comparing a government database (the PA death certificates which include the images of the actual records) with an indexed database of government records that was created by Ancestry (in 2014, Ancestry created index of a previous index of SS-5 Social Security Applications - i,e, Ancestry's index is an "index of an index" - they did not index the original SS-5 forms). The SS-5 entries are the applications that people filled out when they applied for Social Security. The index does sometimes contain additional information such as a date-of-death, but not always.
Now, to be fair, there are many errors of fact in the PA death certificates (especially the information that was given by informants), but, the death dates themselves in the PA death certificates are very reliable. Beyond this, when you look at the image of the death record, you get a host of very specific details on William Hickey that correspond exactly with nearly every other fact on William Hickey that is listed in the SS-5 index entry (name - William Hickey, SS#, birth date of 21 July 1876, birth place of Otsego, NY). The only fact that is different between the PA death record and this SS-5 index entry is the date-of-death.
Just looking at the original PA death certificate from 1960 and the Ancestry SS-5 index (which was created in 2014 I believe), I would strongly lead toward the 1960 death date, and look for a newspaper obituary from a Philadelphia newspaper from 1960 to corroborate this date.
I also strongly suspect that the 21 October 1941 death date (which is also listed as the claim date - i.e. the date when his family filed a claim for his Social Security benefits) is actually the date when William Hickey first sent in his SS-5 application to apply for Social Security benefits. There is an easy (but somewhat pricey) way to check this - you just have to fill out this request for William's original SS-5 form and pay the $27 fee for the photocopy of the original record (note - this is not a time to try and save money by choosing the $16 computer extract, as the computer extracts are indexed records and thus could have an error):
Bottom line - I always trust original records over an index - especially an index that was made in 2014. If you order this, my guess is that you will find that William's SS-5 application is dated October 21, 1941.