"As best as I can explain, its a limited partnership, required to shell out 90 % of its earnings."
That is correct. But more to the point, MLPs are required to distribute 90% of their net taxable income. This sum (cash available for distribution, CAD, or distributable cash flow), can be sigificantly higher than GAAP earnings, which often include non-cash items, such as depreciation and hedges. REITs have a similar "problem," however, most investors in REITs are used to looking at funds from operations (FFO) as a measure of distributable cash flow. This seems not to be the case for MLPs, at least not yet.