That’s probably just a random fluctuation. You can improve accuracy by tweaking the hyperparameters (depth, number of trees, pruning threshold), but you have to be careful about overfitting. You can either setup cross-validation yourself and do a loop over different combinations of hyperparameter values, or use the ScikitLearn.jl interface, along with GridSearchCV to do model selection.
JLD.jl should work for saving pure-Julia structures to disk.