Certain bacteria can live for months on harder surfaces, and while the sealing and capping process should be done under sterile conditions, it's always best to assume the worst. Another technical aspect is how long you swab for, and while hard surfaces can be swabbed quickly and the alcohol left to do its job, porous surfaces like the skin should be swabbed actively for at least 10 seconds, preferably 15-30 seconds, in order to allow the alcohol to do its job and kill bacteria. While most will die in 10 seconds, some do take longer, and it's important that it has its time to work. In this case, 70% isopropyl actually works better than 91%, as the added water content allows the alcohol to penetrate and kill microorganisms by more easily reaching the inside of the cell. These small details may not matter 99.99% of the time, and may have absolutely nothing to do with your issue, but the one time they might, you'll be glad you took the additional precautions.