At least, that's the understanding I got from this source on conventions (clause 4 of the "overcalls" section).
My preference is for 5-5 in the minors or the two lowest relevant suits, and a minimum of eight high card points in the two suits. If the distribution were 5-4, I'd want two extra high card points, a total of 10. Taking this way of thinking to its ultimate conclusion, I would be satisfied with a 4-4 distribution and 12 high card points.
One way of looking at it is that you need 5-5 distribution for such a weak bid. Another way is that 4-4 is "enough" if your high card strength is sufficient to compensate. To me, the 5-4 requirement is "neither here nor there." Put another way, if four cards was adequate for one suit, it should be enough for both suits. It seems like a "compromise" between the two positions. Are there "playing" reasons for such a posture (that is, are people more likely to get into trouble if they use the "4-4" standard that I advocate)? Or was "compromise" the reason this standard was adopted to make things easier for "Basic" players?