• Gurobi Staff

Yes, the difference is probably due to presolve. LP presolve isn't exactly the same as MIP presolve. On the LP:

Presolve removed 199711 rows and 197457 columnsPresolve time: 1.49sPresolved: 62222 rows, 61266 columns, 252112 nonzeros

On the MIP, Gurobi reduces the model even further:

Presolve removed 223213 rows and 221193 columnsPresolve time: 7.64sPresolved: 38720 rows, 37530 columns, 173928 nonzeros
The presolved LP model is ~50% larger than the presolved MIP model and takes awhile to solve. Meanwhile, Gurobi solves the root relaxation of the presolved MIP model fairly quickly, then finds a near-optimal heuristic solution at the root node almost instantly. I agree this behavior is strange, but it makes sense when considering the size differences of the presolved models.