This is quite normal
Because you aren't plugging a 15A space heater into every circuit with the dishwasher, microwave, dryer, A/C, water heater, and stove all running full blast, circuits have two values associated with them: an ampacity, and a rated load. The ampacity is set by the wire in the wall and the wiring devices (receptacles, light fixtures, switches) connected to said wires, and is governed by the circuit breaker in the panel, which will be selected according to the Code rules (Article 240 and Table 310.15(B)(16)) in order to protect the wire and wiring devices from overloads and shorts. The rated load, however, is set based on a series of rules provided in the National Electrical Code (Part II of Article 220, to be precise):
- General lighting and receptacles are provided for by an allowance that covers all such branch circuits (for a residence/dwelling unit, this is 3VA per square foot).
- Kitchen and laundry small appliance branch circuits add 1500VA each.
- Furthermore, there is a demand factor applied to the above loads to account for not everything in your house being on all the time. (You get to halve any of this general receptacle/lighting load that is above 10kVA, in other words.)
- Standard residential electric dryers are counted using an allowance of 5000VA. Likewise, electric ranges get an allowance based on the size of the range -- for all but the largest residential ranges, 8000VA is a safe figure, but some extreme cases may require the full complexity of the Code's procedure for this.
- Finally, other appliances on dedicated circuits get counted as nameplate loads. (In other words, they use the power figure on the appliance's label/nameplate, which will be same label where the UL/ETL/... markings are found.)
Adding all these up gives you the rated load for your service, which needs to be less than the 100A mark in your case given that that is the ampacity of your service. If you tried to turn everything on full blast at once, though, you might trip the main -- this is your clue that you're running too many things for your service and probably don't need to be drawing quite so much electricity at that point in time, or need a bigger service to handle all the loads you have.
As to the rest of your questions
The inside main breaker is simply a shutoff switch, so its ampacity simply needs to be sufficient (not less than the current flowing through it) not correct (to protect anything) -- the real main breaker is the one located outside. As to that weird looking breaker, it's what's called a quadruplex breaker that packs two 240 volt circuits into one pair of breaker spaces.