The maximum tension (Fmax) exerted by the contractile machinery of the muscle is the product of (1) its physiological cross-sectional area (PCSA) and (2) its specific tension (ST). The actual muscle force developed by a muscle is a (nonlinear) function of its Fmax, its active state (i.e. the proportion of bound and unbound actomyosin crossbridges, the number and type of recruited motor units, the firing frequency of those motor units) and its kinematic state (i.e. the length and velocity of the contractile elements).
Specific tension it theoretically constant, so we will presume you have not changed it through training. You have gained no size so we will presume PCSA has also not changed. We will also assume the kinematics of your lifting technique have not changed substantially, so the muscle's kinematic states have not changed (although there is some evidence that resistance training can enhance the force-velocity characteristics of muscle without concurrent increases in muscle mass). This leaves a change in the active state (i.e. neurological factors) as the only explanation for your enhanced muscular capabilities
In addition, your ability to move weight is most directly a function of the net moments your muscles develop at your joints. Forces in protagonist muscles (e.g. the biceps during a curl) will increase the moment while forces in antagonist muscles (e.g. the triceps) will decrease the moment. So it is possible that the actual strength of your individual muscles have not increased, but your degree of antagonistic coactivation has decreased. This possibility could also be placed under the "neural factors" umbrella.
