Why is the app name under /data/app different from that under /data/data?

After I installed DiskInfo.apk through adb install DiskInfo.apk, under /data/app, there is an installed apk named:

me.kuder.diskinfo.pro-1.apk

However, under /data/data, there is a new directory named:

me.kuder.diskinfo.pro

Why is the apk name under /data/app appended with "-1"?

And why is the name under /data/app different from that under /data/data?