73qanda
73Q, I don't think you're a High Capacity pilot or you'd know that you're quoting is what you'll find in every airlines Fuel Policy, and you're taking it out of context!
It doesn't imply you'll have fuel to another port if your destination suddenly requires an alternate due to un-forecast changes, however if you have fuel to another port at the time it deteriorates, your obliged to apply the altn requirement and divert, or if it goes sour you'll answer for it.
The inflight quote ensures you don't continue on to destination after departure if your fuel level reduces to the level below what your min arrival fuel is pre departure. If your burn is > than planned (stronger H/Ws) such you won't have that fuel, you need to divert. E.G. o/head your last suitable A/P you must have burn to dest, app fuel, 10% of both and your FR etc. Less than that and you cannot continue. At some point after you leave behind that last suitable and your destination wx goes below altn min you will obviously not have altn fuel. Or if your last suitable enroute goes to altn as well.
SLAM is based on specific requirements and is available pre and inflt.