I will try a simple way to explain it.
900 cell got layer = 2
900 cell got layer_threshold = 80
900 cell got layer Hyst = 5
1800 cell got layer = 1 (implies higher priority)
1800 cell got layer_threshold = 70
1800 cell got layer Hyst = 5
a MS assign to 1800 cell because it has higher layer priority + Rxlevel is better than layer_thr_1800. MS HO to 900 cell only if Rxlevel is worse than (layer_thr_1800 + layer_hyst_1800)and RXlevel is better than layer_thr_900.
so in your case you mobile stayed in 1800 until level -75 (layer_thr_1800 + layer_hyst_1800).