Multiple updates with same Malware

I was curious as to why multiple updates contain the same definition for Win32/TrojanDownloader.Zortob.B?

For instance, the signature for this piece of malware is included in update 7337, 7338, and 7339.

Also, what is the number in parenethesis after the Malware name?

http://go.eset.com/us/threat-center/threatsense-updates/page/2/



Win32/TrojanDownloader.Zortob.B(47) 7339
Win32/TrojanDownloader.Zortob.B(3) 7338
Win32/TrojanDownloader.Zortob.B 7337

What is the difference?

 

Probably a slight variant of the same family malware that merits separate detection

 

Often the malware remains same, just the packer changes to avoid detection.