Gmail does a good job of building your contacts list from email messages you've sent and received. After some years go by, you tend to have multiple entries based on various email addresses and sometimes names, for various people. If you further add in entries to track, say phone numbers and addresses, it gets even more confusing.
Finally, now with the Android phone, your old cell phone list* gets sucked into the Gmail contacts list, since they are fully integrated meaning one and the same on the Android phone, so now there may be one or more entries for various phone numbers, too.
Gmail Contacts actually offers a neat feature to fix this. If you search on a person's name and get several results back, you can check off all entries that are the same person and click on a Merge Contacts link. That will throw all of that data into one contact. Then you can go in and edit it, cleaning up the name (i.e., names like John Cel and John Work), delete old phone numbers and indicate which are mobile, work or home, delete obsolete email addresses, etc.
Android always offers you a choice of matching phone numbers when calling and you select from the list. Gmail does the same for sending email.
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* I've been a T-Mobile customer for eight or more years (it used to be Voicestream and before that was Powertel) so I've been carrying around the same SIM card all that time. Okay, a couple of years ago I did let the store copy my old SIM card to a newer, larger one. Since the SIM card has the phone list on it, I had all of my numbers every time I switched GSM phones by just plugging it into the next phone.