The prizes are random.
So just as, if you buy a million Lotto tickets, you have more chance of winning than if you only buy one, if you have thousands of bonds you have more chance of winning.
I've forgotten the odds, but (remembering that most of the prizes are £50 and you have negligible chance of winning a million, if the interest rate was 5% (which it isn't) and you invested say £10,000 you would expect to get £500 a year interest, so you could expect about £500 a year prizes on average, so about ten a year on average. But, since it's random, there's no reason why you shouldn't win a hundred prizes one year, and none the next.
But with bigger holdings, it does tend to average out at about the published rate.
The payment rate is set to give an average return of about the same as the interest on a typical savings account, net of basic tax. (prizes are not taxable). So if you are a higher-rate taxpayer it makes a reasonable place to tuck a wedge of money, with the slim chance of winning a million. And of course you can always get your stake back, with little risk of the fund going bust.