There is no mystery about "tracking error" with the leveraged ETF's. They do precisely what they state. But this does not mean that they will be 2x across any time frame.
Consider:
Example: Let's just say the TLT and the TBT (2x inverse of TLT) are both at $10 (for ease of math). One day, the TLT goes from $10 to $9, so the TBT goes from $10 to $12. All well and good. But then, if the TLT goes back from $9 to $10 the next day it is climbing 11.11%... which means the TBT drops 22.22% from $12, leaving it at $9.36. So you are actually not wrong (the TLT has moved from 10 to 10), but you're still down on the play (TBT goes from 10 to 9.36).
As long as the underlying goes in the same direction each day it works as it should, but as soon as you enter a period of chop in the underlying, the leveraged ETF starts to drop because of the math of how it moves.
For a more extreme example, let's say the IYF falls to $2 as the financial system implodes causing the SKF (2x inverse of the IYF) to skyrocket to $10,000 or something like that. Then the Fed comes out and announces some program that causes the IYF to gap up to $3 (50% gain) the next day. the SKF would go instantly from $10,000 to $0.