Apple today said a software problem — and not a faulty antenna design — is responsible for reports of poor reception by some iPhone 4 owners.

Apple admitted its signal strength miscalculation dates back to its original 2007 iPhone. It promised to fix the glitch in a few weeks, but did not directly address concerns that its antenna design causes reception problems for iPhone 4, its newest phone.

Apple's apology -- a relative rarity from the company known for its marketing savvy -- marked the third time in less than three weeks it had to apologize to customers of iPhone 4.

"Upon investigation, we were stunned to find that the formula we use to calculate how many bars of signal strength to display is totally wrong," Apple said in an open letter to customers published on Friday.