A while ago I had this problem where the People search results displayed in the ‘People Search Core Results’ web part & "Search High Confidence Results" web parts didn't show all the Skills from the User Profile. Only the last skill were returned from the search engine. I had the same behaviour when searching for people through the SharePoint search web service.
In the User Profile, as you can see below, multiple skills had been added to the SPS-Skills property. Notice that the last one is "Test Theory".
So when I did a serch for this user, I got the following XML output.
As you can see, only the last value was returned.
So what was causing this?
Well, after a lot of troubleshooting I found that this behaviour can occur if you edit the Skills Managed property using the Search Settings. When you edit this property or simply click the OK button on the Shared Services Administration: SSP > Search Administration > Managed Properties > Edit Managed Property > Edit Managed Property – Skills, the field "HasMultipleValues" within the SSP database table MSSManagedProperties, gets updated. By default the value for this field is 1, and it is updated to 0 even though this field does not map to any user editable properties on the SharePoint user interface.
Microsoft have confirmed this "issue", but do NOT consider it a bug. 
Update: Microsoft have changed their mind and now consider it a bug. No date for a QFE has been set.
Fortunately there's a way to reset HasMultipleValues to 1.
1. Create a custom dummy multivalue User Profile property and add it to the Skills Managed property.
2. Ensure that “Include values from all crawled properties mapped” is set.
3. Click OK.
Now the ‘HasMulitpleValues’ fiels is set to 1.
Do a full crawl. Executing the initial query again now returns all values from the Skills property.