Hmm found the official post, but it's more odd than I thought. Here is a quote from Paizo Employee Josh. link:
http://paizo.com/paizo/messageboards...paizo/eidolons
Joshua J. Frost, Thu, Mar 4, 2010, 02:27 PM
As for the Improved Natural Armor evolution, that last sentence is unfortunately not very clear. Here's the intent and how you should handle it until the class is finalized:
The intent was that you could take it at level 1, level 10, level 15, and level 20. So it should read something like, "This evolution may be taken once for every five levels the summoner possesses, minimum level 1." So you could take it at level 1, but then you'd need ten summoner levels to take it again (level 10), 15 summoner levels to take it a third time, and 20 summoner levels to take it a fourth time.
Hope this helps!