Well, keep in mind that "Assault" and "assault" can be used two different ways. During the era Thunderbolts were probably quite common among assault lances and so ultimately fell into the "Assault" class, being stacked against other common 'Mechs of the day. RATs represent likely combinations or groupings, not necessarily weight all the time.
Remember, you're rolling for random assignments in the assault category. If Thunderbolts were likely to be assigned in the assault category, they should and would be represented.
Just something to think about.