What am I missing about ignition timing?

[71CHEVYSHORTBED402]

I replaced the chain and oil pump on a 402. The marks on timing gears are straight up 12 O-clock, and the harmonic balancer is dead-on zero. I think that means TDC to #1 (ha!). I reinstalled a Delco-Remy HEI. I didn’t mark anything, due to new paint and that, so like starting over I guess.

I turned the slot in the oil pump rod with a screw driver to approx. the #1 cylinder. Then set the distributor forward, marked the distributor where the front LH rotor will face the #1 cylinder, and spun the rotor to the marking.

From there I was able to mesh the gears very easily, goes straight in, but here is where I’m lost. After I mesh the gears, and as I lower it further into the gears - or more likely insert the distributor rod into the oil pump rod, the rotor swings clockwise to approx. straight forward.

Corrections welcome, but I think its right. Edit: The rotor must land on #1. Assuming it’s right, how come the initial rotor position (near forward) doesn’t screw things up? What am I missing?