Observed rate of supernova in the MW

[Hypernova]

I often see it mentioned in articles on supernova that the average rate of supernova in the Milky Way is around 1-2 per century. It is also said that the actual rate of observed SN is far below the expected rate, easily seen as the last naked eye one was in 1604, although the progenitor of Cassiopeia A probably went bang around 1680 and G1.9+0.3 in the mid 19th century, both not conclusively observed at the time.

I can't help thinking that reason why we observe less SN with the MW is simply because of the speed of light. There could well indeed be an average of 1-2 a century somewhere in the galaxy but the light would still need to travel thousands of light years to reach us. Obviously there would be huge variations in light travel time depending on the distance. There is also a huge region on the opposite side of the galactic core that we still haven't seen, even with the current generation of infrared observatories where SN may be occurring. I am confident that the science community have probably taken this all into account but since I have never seen this being touched on in popular science articles I thought that I'd ask here.

Thoughts

[acey]

A study of gamma ray bursts a few years ago deduced a rate of one supernova every 50 years on average within the Milky Way. We haven't been seeing most of them because of absorption by dust and gas, but we may also be overdue one - the two per century rate is only a long-term average.

INTEGRAL reveals Milky Ways' supernova rate - CERN Courier

[badgerchap]

Also, wouldn't the "2 per century" average itself out over a longer, general timescale? Surely the "rule" can be applied as accurately to observations of supernovae just as well as it can be to the actual events?