Yeah I noticed the bevel at the back.
I guess the only reason I can think is that obviously if the block was tilted forwards you would be sanding flatter to the fin, or conversely if you tilted the block back you could allow for even steeper bevels, but I guess building it to sit flat makes the most sense.
I really like the simplicity of your design cutting out power tools, I may try and rustle something up similar if you don't mind. How do you clamp down your fins? I was thinking if you built the base as a 'T' shape it would be easier to put a G clamp around the fin and base.