With no need to get into the details (although the article is linked below if you wish to read it), the coefficient of rolling friction is a measurement of how much the bottom of a wheel deforms when in contact with the ground.
Learning about rolling friction, I've come across what's called the Static Coefficient of Rolling Friction. This coefficient, a measure of the amount of deformation when a wheel is standing still, is supposedly different from the regular Coefficient of rolling friction, the coefficient when a wheel is already rolling.
Why would a wheel deform more or less when rolling across the ground than when not rolling and just standing still? Why are there two different coefficients?
(Any theory is appreciated, doesn't necessarilly have to be the correct anser)