Stripe/Instacart equity grants as fixed $ amount of RSUs?

I recently read that Stripe and Instacart are now giving employees RSUs equal to a fixed dollar amount every year at vesting time instead of a fixed number of RSUs, both for new hires and for refresh grants. (source: online forums, an HN comment)

This was quite surprising to me because it means that employees get no growth potential from future vests, i.e. all growth before a vest doesn't benefit employees (except for any already-vested RSUs).

AFAIK most big tech companies (~AppAmaGooBookSoft) give a fixed number of RSUs in their grants (though there might be a one-time conversion from a dollar amount to decide the number of RSUs).

Any idea why Stripe and Instacart decided to go this way? To protect their employees from the stock price falling? To discourage people from applying just for the IPO boom? Does this save them a lot of money somehow? Anything else I'm missing?