A recent, more sophisticated method of analysis, which has not yet reached the level of an official standard, is to compare the numbers of mature and immature RBCs in an athlete's circulation. If a high number of mature RBCs is not accompanied by a high number of immature RBCs—called reticulocytes--it suggests that the mature RBCs were artificially introduced by transfusion. EPO use can also lead to a similar RBC profile because a preponderance of mature RBCs tends to suppress the formation of reticulocytes. A measure known as the "stimulation index" or "off-score" has been proposed based on an equation involving hemoglobin and reticulocyte concentrations. A normal score is 85-95 and scores over 133 are considered evidence of doping. (The stimulation index is defined as Hb (g/L) minus sixty times the square root of the percentage of RBCs identified as reticulocytes.)
So, assuming this Wikipedia entry is accurate, it sounds like the allegation is that Levi either was blood doping or using EPO.
According to information from Landis's allegations that Levi and Lance Armstrong used blood doping, pro cyclists learned to manage this coefficient by taking small doses of EPO at night after they have a transfusion--aka microdosing. (See this link for more information on microdosing and other methods that cyclists use to conceal their doping activities.) Apparently, the EPO doses are so small that, unless they are tested in the middle of the night, the EPO cannot be detected. Landis said that this creates new reticulocytes, presumably to keep the blood off-score coefficient below the magic 133.
In any case, if the allegation is true, it looks like Levi was right there, barely below 133. What a coincidence!