breaking my routine today

I have a scientific conundrum

     Jeez, I hope I used conundrums correctly.
     We have been in the house for two years, three months.
     Last week I replaced a light bulb in one of our kitchen pendant lamps.  Today I replaced a battery in a smoke detector.
     Earlier today, Jackie asked me what was beeping.  I heard nothing.  I have a ringing in my ears that sounds like crickets and high pitched radio frequencies.  It's a constant.  Goes 24 hours a day, awake, asleep, inside, outside, sober, drunk......constantly.
     Sometimes I will jump up to answer the phone, only to be told it wasn't ringing.  Other times a noise will be irritating wifey and I have no idea, because I don't hear it.
     The basement smoke detector was doing it's chirping sound, telling me it was time to replace the battery.  Of course, we did not have a battery.  After going to the store, I proceeded to start reading the Trib on line and when niece Cindy called, talked to her for a while.  I went back to the Trib when a shrill voice said "Fix that damn smoke detector.  It is driving me crazy."  Since I couldn't hear it, it didn't bother me....but I heard the tone.
     Two minutes later, battery replaced, chirp gone.
     Now my scientific conundrum:  All the smoke detectors were installed at the same time.  All the lights were put in at the same time.
      Why don't all the lights, and all the detectors, go out at one time?  Or even within days of each other?
      They are all on the same amount of time, are all the same age, have the same batteries or light bulbs....yet some last longer.
     Silly me, I only bought 3 batteries.  It struck me I should replace all the smoke detector batteries at one time, on one special day, so I would make it an annual event.
     That would involve planning and rationality, something I am not good at.
     By the way, my routine is changed tonight.  I usually read the Internet, play solitaire, then do my blog but tonight I opted to do the blog first.
     Next I will read the Internet and then take a nice, hot shower that will hopefully relax me and allow me to sleep tonight, perchance to dream.
     Hopefully I won't be up at 3 a.m. talking to people on Facebook.  Not that I don't like doing that, but I am really tired.