what is the diff b/w free testosterone vs total testosterone??

Google is your friend.

"Most circulating testosterone is bound to sex hormone-binding globulin (SHBG), which in men also is called testosterone-binding globulin. A lesser fraction is albumin bound and a small proportion exists as free hormone. Historically, only the free testosterone was thought to be the biologically active component. However, testosterone is weakly bound to serum albumin and dissociates freely in the capillary bed, thereby becoming readily available for tissue uptake. All non-SHBG-bound testosterone is therefore considered bioavailable."

http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/83686

I'm not sure what the "why" you're asking is in regard to. Why is some of the total testosterone free and some bound? As GrnTea explained some of the testosterone in the bloodstream is unbound (1-2% I believe) and the rest is either "loosely" bound to albumin or bound to SHBG.

Globulins and albumin are transport proteins synthesized by the liver. Transport proteins improve the transportability of a lipid-soluble hormone and serve as a reservoir so that a normal concentration of free hormone can be maintained. Hormones that are bound to a transport protein are "protected" from being broken down by enzymes and kidney filtration and have a longer half-life than free hormones.

I have no idea what can lead to a low concentration of free hormone despite a normal total testosterone. Many medical conditions and some medications affect testosterone levels as well as age, time of day (usually higher in the morning if I remember correctly).