could anyone help me interpret these?

My doc says I have very mild asthma (based on 13% reduction in FEV).
But the 50-75% numbers are very low, they are affected severely (up to
3-fold change) by steroid/nebulizer treatments. I wonder what the
FEF50-75 ## mean (small airways?) for the asthma diagnosis, and why
don't asthma docs ever mention them to me. My peak flow (proportional
to FEV1?) never changes, no matter how bad I am feeling.

test after prolonged Advair treatment (for reference):

FVC 5.69 L (~95%)
FEV 4.44 L (~100%)
FEV/FVC 0.78
FEF 25-75% 4.59 L/sec (~100%)
FEF 50%    5.91 L/sec (~100%)
FEF 75%    2.53 L/sec (~100%)

test with no inhaled steroid for 48 hrs prior to tests
Pre-nebulizer:

FVC 6.57 L (115%)
FEV1 4.21 L (97%)
FEF25-75% 2.47 L/s (54%) - a ~50% reduction compared to steroid case
FEF 50% 3.69 L/s (62%) - a ~30% reduction compared to steroid case
FEF 75% 0.80 L/s (32%) - a 3-fold (!) reduction compared to steroid
case.
NOTE: I had similar (low FEF) numbers before I have developed any
asthma symptoms at all (obtained during a routine, non-asthma related,
doctor visit).

Post-nebulilzer:

FVC 6.79L (119%, 3% increase)
FEV1 4.86 L (110%, 13% increase - the only # discussed by my doc)
FEF25-75% 3.52 L/s (77%, a 43% increase)
FEF 50% 4.79 L/s (81%, a 30% increase)
FEF 75% 1.21 L/s (48%, a 52% increase)

Any help would be greatly appeciated. I can only see my doc once a
month, and I can't call/email him.