Help understanding a full wave bridge rectifier
Both of your questions are answered by this simple fact: When a diode is conducting, it will have a voltage drop across it. For your standard silicon diode (1N4148, 1N4001, etc), this is somewhere around 0.5~0.7V under normal conditions (it increases slightly at high currents). For Schottky diodes, it's closer to 0.2~0.3V, germanium diodes are around 0.3~0.4V, and very high power diodes can be very high, up to multiple volts or even tens of volts.

Since in a bridge rectifier, two diodes are conducting at any one time, you'll be losing two diode drops worth of voltage; this explains your reduced voltage. The flat bits are a consequence of this as well: if there isn't enough voltage to turn the diodes on, they simply won't conduct.