Saflager 34/70 under attenuation 67-69%

[StenbergBrewing]

Has anyone experienced consistent under attenuation with 34/70? I'm on my 4th batch and all are testing between 67%-69% attenuation. The beers taste good (not sweet or worty) but the numbers don't seem to be matching what others suggest as proper attenuation. My ph's have all been good in the mash, I oxygenate 1 min, I pitched the proper amounts from mr malty on my first batch but may have over pitched in the others as I re pitched slurry into them. The only other thing I can think of is my mash times. My first batch was an experiment of 20 min mash. The other batches ranged 30-45 min. Others have done this in past experiments but I never hear anyone reporting under attenuation. Thoughts?????

 

[Gnomebrewer]

My attenuation with 34/70 is always around 80%.

Could it be a measurement error (dodgy thermometer or hydrometer)?
or
Beta amylase works much slower than Alpha amylase; it's possible with these short mashes that you are ending up with more Alpha conversion (less beta conversion) than you would with a longer mash, meaning a less fermentable wort. That's only a guess though - no experiment to back it up. Also, I think short mashes will work best if you crush really fine (eg. BIAB).