These values come from the assignments for the expert severity and groups levels, which are defined in proto.h.
So for example, PI_ERROR is defined as having the hexadecimal value of 0x00800000, which is just the decimal value 8,388,608, and PI_MALFORMED is defined as having the hexadecimal value of 0x07000000, which is just the decimal value 117,440,512.
Here's the full list (as of this writing), copied from proto.h for convenience and with decimal values added in comments:
#define PI_SEVERITY_MASK 0x00F00000 /* mask usually for internal use only! */
#define PI_COMMENT 0x00100000 /* 1,048,576 */
#define PI_CHAT 0x00200000 /* 2,097,152 */
#define PI_NOTE 0x00400000 /* 4,194,304 */
#define PI_WARN 0x00600000 /* 6,291,456 */
#define PI_ERROR 0x00800000 /* 8,388,608 */
#define PI_GROUP_MASK 0xFF000000 /* mask usually for internal use only! */
#define PI_CHECKSUM 0x01000000 /* 16,777,216 */
#define PI_SEQUENCE 0x02000000 /* 33,554,432 */
#define PI_RESPONSE_CODE 0x03000000 /* 50,331,648 */
#define PI_REQUEST_CODE 0x04000000 /* 67,108,864 */
#define PI_UNDECODED 0x05000000 /* 83,886,080 */
#define PI_REASSEMBLE 0x06000000 /* 100,663,296 */
#define PI_MALFORMED 0x07000000 /* 117,440,512 */
#define PI_DEBUG 0x08000000 /* 134,217,728 */
#define PI_PROTOCOL 0x09000000 /* 150,994,944 */
#define PI_SECURITY 0x0a000000 /* 167,772,160 */
#define PI_COMMENTS_GROUP 0x0b000000 /* 184,549,376 */
#define PI_DECRYPTION 0x0c000000 /* 201,326,592 */
#define PI_ASSUMPTION 0x0d000000 /* 218,103,808 */
#define PI_DEPRECATED 0x0e000000 /* 234,881,024 */
