libpcap format - packet header - incl_len / orig_len

The libpcap packet header structure has 2 length fields:

typedef struct pcaprec_hdr_s {
        guint32 ts_sec;         /* timestamp seconds */
        guint32 ts_usec;        /* timestamp microseconds */
        guint32 incl_len;       /* number of octets of packet saved in file */
        guint32 orig_len;       /* actual length of packet */
} pcaprec_hdr_t;

incl_len: the number of bytes of packet data actually captured and saved in the file. This value should never become larger than orig_len or the snaplen value of the global header.

orig_len: the length of the packet as it appeared on the network when it was captured. If incl_len and orig_len differ, the actually saved packet size was limited by snaplen.

Can any one tell me what is the difference between the 2 length fields? We are saving the packet in entirely then how can the 2 differ?