As written, there can be a lot more than only 10 XP accumulated after a session.
The cost for advancements of Attributes, Skills, Focus fields and Talents are all the same, 10 XP.
I understand that keeping it to a single advancement between sessions allows for keeping the power level of PCs at about the same for every player in the group.
Allowing for spending XP on multiple advancements could lead to some characters becoming much more powerful in a short time, while others stay behind. That combined with the competitive aspect of voting for the “most valuable player” might frustrate some players sooner or later.
I like keeping all PCs at about the same advancement/power level, as that makes it easier to set up challenges for the group. Considering the often quite impressive effect most Talents offer, having a character who is 4 Talents “ahead” of the others makes that one significantly more competent.
Having variable costs does only slow the different speed of character progress a bit down, but does require individual bookkeeping of individual XP values accumulated and spent.
I prefer to avoid this kind of bookkeeping entirely. In my Savage Worlds, D&D 5E, Shadow of the Demon Lord games, I award an advancement / level-up per adventure. That is easy to manage and keeps every one at about the same level of competence.
Individual acts of heroism, great roleplay, etc. I like to reward by immediately awarding Fortune points. This direct, timely feedback is to me preferable to awarding additional XP to be spent much later or - considering the single advancement - never at all.