@Dollywilde Those are good points. But also, looking at a spread of 100 women is different from looking at the chances of each individual woman each month. When looking at it from an individual woman's point of view, the average woman has a 30% chance of getting pregnant each and every month.
So it's a bit like if you flip a coin. You choose Tails, and then you are unlucky enough to get Heads four times. It's unlikely, but it could happen. Now, on the fifth flip, you might be feeling like you are definitely going to get Heads again because you've already lost four times - it's hopeless. Or you might feel like you'll definitely get Tails next time, because you've already had all those Heads. But your actual chance of getting Tails is still 50%, for each individual flip.
So if you get:
Month 1: Not pregnant
Month 2: Not pregnant
Month 3: Not pregnant
etc
Your chance for Month 4 (for the average woman), is still 30%, the same as the first month.
Some women will get lucky on the first roll of the dice and get pregnant in month 1. For those who didn't get lucky, they roll again with that same chance they had in month 1 - so some more get pregnant. If an average woman reaches month 6, they've just been less lucky than the women who got pregnant in months 1-5, and in the next individual cycle, they have the exact same chance as the previous month and as in month 1: 30%.
And if they reach month 12, they may still have just been really unlucky, and just need a few more rolls of the dice, and each roll will still have that 30% chance.
Of course, for each individual woman, we don't know the exact percentange chance for them each month: it depends on age, underlying conditions, partner's age and health, environmental factors, etc. If a woman is younger and has no underlying issues, the chance is likely greater than 30% and so more likely to get pregnant with each try and less likely to reach 12 months. If older and e.g. the woman has PCOS, then it's lower than 30% chance each try so the woman is more likely to reach 12 months. But for most women, it still comes down to luck.