Ryan Phillippe's ex-girlfriend's accusations -- that he brutally attacked her and threw her down the stairs -- don't add up ... according to cops.

Law enforcement sources tell TMZ ... investigators don't believe Elsie Hewitt's account of what went down between her and Phillippe on July 4 for a couple reasons. First -- they say Ryan was shocked when cops showed up at his place after Elsie filed a police report ... but was completely cooperative.

We're told he invited them in and answered all their questions without lawyering up -- like he had nothing to hide.

Next, there's Elsie's story about the stairs. Our sources tell us the stairs she accuses Ryan of throwing her down twice are sharp and wooden, and would have left her with much nastier injuries -- even broken bones -- than what she laid out in her suit.

As we reported ... the L.A. City Attorney refused to prosecute Phillippe after investigating, but Hewitt still sued him. Ryan, in response, has hired a lawyer and plans to go after Elsie for defamation.

Phillippe himself is making it clear he has nothing to hide, saying, "At the time these allegations were initially made, I fully cooperated with law enforcement and a thorough investigation was conducted."

He adds, "Domestic violence is a very real and tragic issue faced by many women the world over and should never be used to vengefully slander or as a ploy for monetary gain."

"Everyone of my accuser's allegations are false."