To be honest this makes sense to me, a couple of things need to be factored in that make the 52k a better option initially
1. Planning permission, the smaller capacity and design innitally of the proposal on an iconic site, the more likely it is to get granted
2. It meets the needs of investors/funders a smaller site mitigates risk to investment, but also has the appeal of maximizing profit should the market prove to be there.
3. Innitial cost, a smaller capacity is likely to be lower.
4. Future proofed, can be increased up to 60k+, should the market prove there and capacity can be increased.
5. Mitigates against risk against not getting crowds of 60k.
Things are never black and white and you need to accommodate flexibility, i would have been disappointed with a lifetime limit of 52k, but the fact the designs are future proofed up to 62k, its likely the best outcome holistically.