Well - Subaru said it was normal since the dealer looked at several cars and they were the same. I replaced them with bulbs from NAPA at the end of August, and they are still perfectly clear, so I disagree with Subaru's claim that it's normal. (Sylvania replaced the bulbs without question - I just had to send the receipt for the replacement ones I bought. They also asked me if I would send them the bad bulbs (after I had the replacements from them - not as a requirement to get replacements) which I did.
the theory put forth was that the bulbs run at partial power, so they are below normal operating temperature - I certainly understand the rationale behind that argument, but then my replacements haven't browned out, so I dunno...