Such as this quote
“Adjustment of this flux value for decay of the neutron source was later introduced and was based on the physical half-life of 238Pu. The current findings would suggest, however, that a more appropriate value for the effective half-life for the total body irradiator is 141.7+or-2.5 y. In addition, variations in the induced counts for a Mn standard on a yearly basis indicate that seasonal differences of approximately 0.5% can be present between the winter and summer months”
The physical half-life is the book/cd-rom value
The effective half-life includes the experimental errors & other stuff, note a SEASONAL component! So I guess my cosmic-ray idea is likely one of the actual errors that leads to a typical effective half-life, look it up further