Unlike erythrocytes that have a very rigid shape and almost cannot change their size (hence the size distribution is indicative and can be used for diagnostic purposes in medicine), lymphocytes can change their size in a wider range, this is why you see the numbers 6-9 and 10-15 μm.
And they indeed cluster into several different groups: so-called "large granular lymphocytes", also known as NK-cells or "natural killers" (usually >10μm) and "small granular lymphocytes", constituted by a large family of T- and B-lymphocites(usually <10μm). But this clustering is not really distinct, for as I said above, both NK and T/B-cells can shrink (if the osmolarity of the external medium grows due to acidosis, inflammation etc.) and swell (on binding many IgE/IgG complexes, on certain cell factor released etc.). Besides, there are also some intermediate size cells, called NKT-cells, that also flatten the distribution.
So, what you can definitely say is that the typical size distribution of lymphocytes has two peaks: around 8 and 12 μm respectively.