For those not familiar with Ada and bit representations, the compiler is exploiting the fact that types in Ada can have constraints that limit their size.

T1 is a type of integer ranging from 16 to 19 (inclusive). Thus, it can only be four distinct values (16, 17, 18, 19). Two bits is enough to represent these four distinct values and thus the binary representation of the type (in record R) can be reduced to 2 bits.