The veteran German has lost his place to Almunia this season and has started just one game for the Gunners since August.
He has recently admitted that he is prepared to stay at Emirates Stadium, despite fears for his place in the Germany side for Euro 2008.
However, Lehmann concedes that he is still frustrated by the situation and has suggested manager Arsene Wenger treats his goalkeeping rival differently.
"My coach confirmed my impression that he uses a different measuring stick to evaluate Almunia," Lehmann told Kicker magazine.
"For me this was a disappointment.
"It's very frustrating. When I see the performances on the field, I get angry and I have to clench my fist in my pocket."