Is it common to be in 12.4/5 100m shape one week and then 12.8/9 100m the next week?

Yes, temperature, wind, rain, humidity, and the previous day/weeks training can all effect times in 100-meter work. Also, depends on whether you are doing a watch on your wrist or have someone else timing you and if that is the same person every week.

Thanks for taking the time provide a thoughtful and helpful answer; if it helps, the conditions were very much the same. Wind and temperature were similar, I was timing myself with a stopwatch on both occasions. I think some of it could have maybe been measurement error from me last week but I try to be as far and honest as possible when timing myself. Would greatly appreciate any more informed opinions on this =)

Look at what Olympic level timers got as handtimes in those Lindy 100s in that link.

You'll see how inaccurate even the best of handtimes can be let alone ones by a runner timing himself over a course he thinks is accurate. It's not about being honest it just hard to do.

The 100 is a technical event. Sprinters who also enter the 200 or 400 tend to be more consistent than those who focus on only one of the three. This is because the second half of the 100 involves pacing, but the entire 400 and more of the 200 require that. First 50 is either run all out, or you run one body length at most behind the lead to have a chance to win by maintaining speed.

So for you timing this its a bit different, you might have just been more excited the first time and ran it like a race. However you can work on this, trial 150 meters next time, if you are still feeling fresh trial 100 too. Either way, when you rest and then trial only the 100 metrs next workout, should be faster. This way psychologically if you trail slower on the 100 well it was after a loss so no matter, but still you have worked on technique and pacing.