Ashton was found guilty of biting Northampton prop Alex Waller during Saracens' 27-12 Premiership victory.
The Rugby Football Union called Waller's evidence "consistent and compelling", but described Ashton's version as "less convincing".
Saracens have 24 hours to decide to appeal against the panel's decision.
The panel's full judgement was released on Thursday, two days after Ashton's punishment was announced.
The 29-year-old former rugby league player denied he had bitten Waller, arguing that he had twisted his head after the player's arm was lodged in his mouth in a ruck.
The panel was shown photographs of Waller's injury, taken by the Northamptonshire physiotherapist, before unanimously deciding to ban Ashton.
"It is the panel's view that what can be seen on the footage is the player taking the opportunity to bite the arm, which had lawfully come to be in the area of his mouth," the RFU said.
"The panel found the player's version of why he was moving his head in a particular way, and the reasons why his mouth remained on the arm, less convincing.
"The panel concluded that the account from the player was very much less plausible."
Ashton was banned for the minimum of 12 weeks with a further week added because of his previous bans.
He was banned for 10 weeks last season for eye-gouging, following a four-week ban for hair-pulling in 2011.
Australia wing Drew Mitchell said on Twitter that Ashton had no excuses for the bite, calling him the "Luis Suarez of rugby", drawing a comparison with the Barcelona forward who has been banned for biting three times.
Rugby union bosses have said they banned Saracens winger Chris Ashton for 13 weeks after finding his description of events "less plausible".