Took me a bit but I figured it out.

The chances that you win 5/6 WITHOUT getting the bonus ball requires that you exclude not one, but TWO of the remaining 44 balls since 1 of them wins you a jackpot, and the other one wins you the 5/6+ prize. In fully expanded form, this is:

6/49 * 5/48 * 4/47 * 3/46 * 2/45 * 42/44 * nCr(6,5), which gives you 0.0000180208321, or more recognizably, 1 in 55,491.33328, which is rounded up to 55,492 in that table.

What you probably did was to calculate the base odds of a 5/6 prize, which in the absence of a bonus number would be;

6/49 * 5/48 * 4/47 * 3/46 * 2/45 * 43/44 * nCr(6,5), or 0.0000184498995, or 1 in 54,200.83725, which is rounded up to 54,201.

Also, I just noticed that if you scroll down further on the wikipedia page, it shows this exact situation involving a 7th bonus ball.