Note the word: "stress"

http://www.powerstream.com/SwPSFAQ.htm

Mean time before failure. This is a statistical calculation, not a
measurement. All the parts in a device have mean time between failure
ratings. These are mathematically combined to get an MTBF number, often
dependent on the stress put on them. Even the via holes in a circuit board
have MTBF ratings. As you might think, as a rule of thumb, the more parts
the less the MTBF. However, sometimes you can replace one part with a few
more reliable parts, or spread the stress of one part over several parts and
increase the MTBF. The human body has a trillion cells and each cell has an
MTBF of 6 years. No wonder I don't feel so good. By the way, PowerStream
engineers are well equipped to calculate MTBF using MIL-HDBK-217E.