Why is myburine foamy even when no protein leakage is there?

[jram2419]

I've been noticing on and off foamy urine during the last six months. I don't have any other difficulties with regards to urination like pain,blood etc.  I had my urine tested 5 months ago and the results were normal. I again tested recently and the results are below. Could someone please give me some guidance on what could be the reason?

SG: 1.005
pH: 6
Protein : absent
Glucose : absent
Ketones : absent
Bilirubin : absent
Urobilinogen : absent
Blood : absent
Nitrite : absent
RBC: 1/hpf
Leukocytes: 1.3/hpf
Crystals: absent
Casts : absent
Bacteria: absent
Yeast : absent

[jram2419]

Can someone shed some light on this?