It doesn't. The only way an ice sheet gains mass under any normal circumstance is new snow falls on top. As more and more snow accumulates over time, the ice sheet grows thicker.
What offsets that is that ice flows under pressure, so the entire sheet has a tendency to head for the ocean. Things are a bit complicated in Antarctica since some of the landscape where the sheet is grounded is below sea level. There are concerns that the seawater is getting warm enough to cause melting there, increasing the flow rate.
If, in fact, ice sheet mass is increasing, that's great in terms of sea level rise. There's no other way to spin it.
What offsets that is that ice flows under pressure, so the entire sheet has a tendency to head for the ocean. Things are a bit complicated in Antarctica since some of the landscape where the sheet is grounded is below sea level. There are concerns that the seawater is getting warm enough to cause melting there, increasing the flow rate.
If, in fact, ice sheet mass is increasing, that's great in terms of sea level rise. There's no other way to spin it.