BJT common collector amplifier with a voltage gain sligthly higher than one
So I think the equations governing your circuit are:

V_R1 = R1 × I_R1

Vb = R2 × I_R2

V_R1 + Vb = Vcc

I_R2 + Ib = I_R1 + I_in

Solving that first group gives:

Vb = (R2 × Vcc + (I_in- Ib) R1 R2 ) / (R1 + R2)

moving on to the rest of the circuit

Vc = Vcc - Ic × R3

Ve = Ie × R4

Ic = hfe × Ib

Ie = Ic + Ib

Ve = Vb - 0.7

Solving that lot I get

Ve = ((1+hfe) R4 (I_in × R1 × R2 - (R1 + R2) × Vbe + R2 × Vcc)) / (R1 × R2 + (1 + hfe)(R1 + R2) R4)

Plugging in your values and solving for steady state (I_in=0)

R1->22000, R2->10000, R3->1200, R4->500, Vbe -> 0.7 , Vcc -> 15, hfe->100

gives:

Ib=69uA, Ic=6.9mA, Ie=7 mA, Vb= 4.2V, Vc = 6.7V and Ve=3.5 V

Those all look plausible and the transistor isn't in saturation,
solving for I_in = (Vg - Vb) / Rg and taking deltas to get the
high frequency limit I get

ΔVe = 0.86 ΔVg

which presumably tallies with your calculations. 

The gain of the circuit doesn't change much with hfe, or the resistor values, so it's unlikely to be that. 

My guess would be that there was a problem with your experimental set-up. The input impedance of this circuit is much higher than the output impedance, and you're dealing with exceedingly small signals (~10mV RMS). At those values you'll have to take a great deal of care that your measurements are not drowned out by resistance is the ground or power cables. Or indeed that there is no coupling of the output back into the input.

The other obvious posibility is that your input is not well approximated by a voltage source in series with a resistor but by a current source. In that case I_in = Vg/Rg and you get (in the limit)

ΔVe = 6 ΔVg.