Hello again,
Precious information is contained in the FLUKA courses material! 
Your pattern is due to the use of not-optimal transport thresholds for e-/e+/gamma for a USRBIN
with such a fine binning as yours. The transport threshold should be set such as the range of your particles in your material (air) is smaller than the width of your bins. Please check out the lecture below:
You were using the default thresholds of the PRECISIO defaults: 100 keV for e-/e+ and ~33 keV for gammas. The e- range for such energy in air is of the order of 13 cm! And your bins are of the order of 1 cm width.
If you lower your e-/e+ threshold to 10 keV (range ~0.24 cm) and that for gammas to 1 keV, then your pattern should disappear (let me know if it does not). You can do so by means of the EMFCUT card. Do not forget to reduce also the production thresholds to the same level.
My advice is not to go to a finer mesh than necessary and then set your thresholds accordingly. Note that everything said in the previous reply still applies.
Many thanks to H. Vincke and C. Theis for the offline enriching discussion.
Hope this helps,
Francisco
PS: Thresholds are important! See below an example of what one manages to get when messing around with thresholds:
