They used to just list a number, but those cards were incredibly broken when played in other formats. There is no way to change a starting life total once the game has actually started, but different formats have different starting totals.
Take Serra Ascendant which is a 1/1 creature with lifelink for . It has an ability, " As long as you have 30 or more life, Serra Ascendant gets +5/+5 and has flying." This ability is incredibly powerful in Commander/EDH or two headed giant(2HG), since if Serra Ascendant is your first turn play, it's a 6/6 with flying and lifelink for a single mana! Now if they had written it as "As long as your life total is at least 10 higher than your starting life total [...]" Serra Ascendant wouldn't turn on until your life total was 50 in EDH or 40 in 2HG, taking away that immediate power it has in those formats.
Felidar Soverign has the same problem, it's a win condition if you have 40 life or more at the start of your turn. That's the starting life total in EDH, and only 10 points higher in 2HG. Getting a out in those formats quickly isn't impossible, though it will take a few turns, all you have to do is maintain your life total, a lot easier than gaining 20 life. Now if it had been written as "At the beginning of your upkeep, if you have double your starting life total, you win the game." it would take 80 in EDH and 60 in 2HG, bringing back the balance it had in the usual constructed formats.
Remember Commander wasn't designed by Wizards of the Coast, it was originally called EDH and made by players, only adopted as an official format when it became popular, these cards were designed with only 20 point starting life totals in mind. Vanguard was never played very heavily, though players in that format could have a starting life total from 12 (Ashnod, at -8) to 38 (Gix, at +18) and was pretty much a failed experiment as far as formats go, no one looked at it for card design. Once EDH became pretty much the most popular format for Magic, they had to start considering starting life totals that were higher than 20 for card design, that meant no more talking about life totals as static numbers, but relative to whatever that different starting point would be.