All BJTs have what is known as "miller capacitance". It can be "altered" by a greater or lesser reverse bias voltage across the collector-base region. Note that C1 shunts this small capacitance to ground and that C3 returns this capacitance to the positive rail thus ensuring miller capacitance is effectively in parallel with the inductor.
This change in capacitance alters the tuned circuit in the collector and shifts (or deviates) the frequency. Given that the average DC voltage on the collector is pretty much 9V (due to the inductor), moving the base voltage up and down modulates the carrier.
Also, pretty much any reverse biased diode I've come across exhibits this effect and that is how varactor diodes work as well.