Medical sales rep Schreiner opted to leave The Bachelor on Monday night's episode despite receiving a one-on-one date card from Ben Flajnik in his hometown of San Francisco.
The 26-year-old from Colorado declined the date and then tracked down Flajnik at his group date's after-party to tell him that she was going home because the experience just wasn't right for her.
During an appearance on Ryan Seacrest's KIIS-FM radio show, host Harrison was asked what was behind Brittney's decision to leave.
"I still don't think she knows," he replied. "I think she just kind of talked herself into leaving. I don't know why she wouldn't at least go on the date to see if it would even work with Ben. I think she just got caught up in the jealously and felt she didn't belong.
"She ended up leaving before she'd even gone on a date which is a Bachelor first - to leave before you even know if you have a chance with the guy."
Schreiner had made an impression on the first night when she arrived to the first cocktail party with her grandmother Sheryl.
However, on receiving the one-on-one date card she admitted that she was feeling "torn and confused" rather than "thrilled and excited" and told viewers that her heart just wasn't in it.
Breaking the news to Flajnik, she said: "I got your card about the one-on-one date and it was the best feeling in the whole world. I feel like I don't deserve a chance with you. I think you'll find a great woman here, it's just not me. I don't want you to waste a one-on-one date in San Fran."
Flajnik admitted that he was "surprised" by her decision, describing it as a "curveball" and saying that it made the whole experience more real for him because it reinforced the fact a woman he liked could leave at any time.
This week's episode of The Bachelor also saw Erika Uhlig and Jaclyn Swartz sent home at the rose ceremony, while former The Bachelor contestant Shawntel Newton - who appeared on Brad Womack's season - was equally unsuccessful.
Newton arrived midway through the cocktail party in a bid to join the race for Flajnik's heart after briefly conversing with him before he became The Bachelor, but the winemaker opted to send her home.
The Bachelor airs Mondays at 8/7c on ABC.
Listen to Chris Harrison talking to Ryan Seacrest below: