403s appearing in WMT but FETCH as GOOGLE returning same URLs as 200

HI there over the last few days WMT has been showing me 403 errors for a series of URLS. However when i FETCH them they come up as 200 OK and when i look in google using keywords they appear in the index. Everything seems fine except for the errors. Is there any reason for this? Is there anywhere i should check for the cause of this. Its a wordpress site @ www.mutantspace.com. Here are a sample of URLs:

http://ift.tt/1wqxs0u
http://ift.tt/1BH6gSZ

thanks Moray