I'm not doubting the discipline or training of either side, but am wondering if the reason may be due to the British Army taking more of a peacekeeping role in the conflict, rather than actively hunting down rebels.
Its a mistake to view the troubles as simply a war between the IRA and the British Army. The issue was more complex than that. It was primarily a conflict between the republican (IRA, INLA, etc) and loyalist (UVF, UDA, etc) sides.
The IRA would view its actions as protecting catholics from loyalist gunmen like the UVF. From that perspective, the casualties were much more "even".
The British Army would deny that it was in a war, but rather a policing action. This limited its rules of engagement. But an IRA "volunteer" knew that the outcome for them was likely death or imprisonment. While this is still an active topic of politics, not history, the British security forces had other options than purely 'military' ones: collusion with loyalist groups for example.