//unlagged - attack prediction
// made this non-static for access from cg_unlagged.c
void CG_ShotgunPattern( vec3_t origin, vec3_t origin2, int seed, int otherEntNum ) {
	int			i;
	float		r, u;
	vec3_t		end;
	vec3_t		forward, right, up;

	// was it a shotgun attack?
	else if ( ent->weapon == WP_SHOTGUN ) {
		// do we have it on for the shotgun?
		if ( cg_delag.integer & 1 || cg_delag.integer & 4 ) {
			int contents;
			vec3_t endPoint, v;

			// do everything like the server does

			SnapVector( muzzlePoint );

			VectorScale( forward, 4096, endPoint );
			SnapVector( endPoint );

			VectorSubtract( endPoint, muzzlePoint, v );
			VectorNormalize( v );
			VectorScale( v, 32, v );
			VectorAdd( muzzlePoint, v, v );

			if ( cgs.glconfig.hardwareType != GLHW_RAGEPRO ) {
				// ragepro can't alpha fade, so don't even bother with smoke
				vec3_t			up;

				contents = trap_CM_PointContents( muzzlePoint, 0 );
				if ( !( contents & CONTENTS_WATER ) ) {
					VectorSet( up, 0, 0, 8 );
					CG_SmokePuff( v, up, 32, 1, 1, 1, 0.33f, 900, cg.time, 0, LEF_PUFF_DONT_SCALE, cgs.media.shotgunSmokePuffShader );
				}
			}

			// do the shotgun pellets
			CG_ShotgunPattern( muzzlePoint, endPoint, cg.oldTime % 256, cg.predictedPlayerState.clientNum );
			//Com_Printf( "Predicted shotgun pattern\n" );
		}
	}

The only possible reason I can imagine is that at some point the unlagged code uses different data/function/calculation than the normal weapon code. Getting hits behind you indicates that the wrong vector component of the view angle is manipulated to calculate the position of hits. I see one diff between the relevant code of 'ShotgunPattern' in 'g_weapon.c' and that of 'CG_ShotgunPattern' in 'cg_weapon.c':

AngleVectors (origin2, forward, right, up);

In cgame you derive the angle vectors from data sent by the server. But unlagged manipulates the player state to do reconciliation