Triggered abilities and replacement effects are processed differently.
A triggered ability looks for the moment when an event happens. When it happens, it triggers, then is put onto the stack. Because a triggered ability does not affect the game immediately, it has no need to be processed at the same time as an event. That's why most triggered ability will look at the game state just after each event to see if they trigger. In particular, this is true for enter-the-battlefield triggered abilities: you look at the game state just after an object has entered the battlefield to see if that triggers anything. (Notable exceptions include leaves-the-battlefield triggers, because for those we care about what objects looked like on the battlefield, so the rule is that we look at the state just before the event to see what triggers).
So, coming back to
Fathom Mage, which has a triggered ability (recognizable by the word “when”), you look at the game state just after it enters the battlefield. Look! It just acquired two +1/+1 counters, so it triggers.
Replacement effects are different. Because they change what an event looks like, we need to know before that event happens how it will be modified in order to carry it out correctly. This is even more important as that modified event can be itself subject to other replacement effects, which we still process before the event happens. Once all replacement effects have been processed, the modified event happens. Remember that “X enters the battlefield with N +1/+1 counters” and “X enters the battlefield tapped” are replacement effects; it will be important below.
It gets a bit more complicated for objects which enter the battlefield. After all, if
Scarwood Treefolk's ability worked only on the battlefield, it would enter the battlefield untapped? No, it's not the desired outcome, so the rules make it so this kind of ability can affect how the object carrying it enters the battlefield. More specifically, a static ability on an object that modifies how objects enter the battlefield will affect its own entering the battlefield if and only if the ability mention the object itself. It the ability mentions a group of objects (for example, “creatures enter the battlefield with a +1/+1 counter”), then it will never affect itself. This is because such an ability makes sense without affecting the object itself, so no need to depart from the general rule of “it had to be there before the event.”
Coming back to
Corpsejack Menace, it has a replacement effect (recognizable by the words “if… would… instead”) that affects a group of objects (“creatures”). Because of this, it will not affect its own entering the battlefield. So it enters the battlefield with just 2 +1/+1 counters, assuming
Master Biomancer's power is 2.
Daniel Kitachewsky
L3, Paris, France
Rules NetRep