First, students in the U.S. often have problems in math and quantitative measurements. It is something that even confuses doctors. They may need more intensity to learn these concepts. It can be tough if you can't ask questions, receive clarification, or have someone who can go back on a problem.
There could also be a difference between those who opt for an online versus ground based course. If the student doesn't live in the area, has a busy schedule, or doesn't like to drive the campus they may experience more distractions that limit their ability to study.
Furthermore, it is also possible that the delivery methods used in the course are not as strong as they should be. Many public universities have only recently adopted online education and may not be able to move out of their traditional roles to change their learning style or implement technologies (like online math tutorials) or office hours to help students get over the "math hump".
Thus, online learning must be designed well to maximize its benefits or otherwise there may be risks in learning effectiveness. However, those who can go through an online educational course, receive high grades, and maintain their persistence have show some positive traits when compared to those in a traditional setting that received a similar grade.
Verhoeven, P. & Wakeling, V. (2011). Student Performance In A Quantitative Methods Course Under Online And Face-To-Face Delivery. American Journal of Business Education (Online), 4 (11).