A/an before a word is dependant on how the subsequent word is pronounced, not spelled. So for that sentence, the implication is that it’s pronounced closer to “erb”, thus “an” to precede instead of “a”. Another example that’s a bit counterintuitive is “one” being pronounced like “won”, so you’d get “a one time thing” rather than “an one time thing”.
After 10,000+ hours grinding, MapleStory's first level 300 player slams the brakes at 299.99 to rant about the MMO and then quit, all on a dev-promoted stream (www.gamesradar.com)
Kotaku EIC Resigns Over New Editorial Edict - Aftermath (aftermath.site)