Note that the 'mtime' field only comes from the DOS-format time that's stored in the zip file, which is only accurate to the nearest 2-seconds. Zip files can store mtime's in multiple, optional fields but PHP's zip library does not retrieve them (nor does it provide API to access the extra fields)
The end result is that you will see timestamps that could be out by one second (if the original timestamp was odd), and won't necessarily match was other unzip programs will show the mtime as.