Strange memory gap after enabling pae

[st1905]

SYSTEM MEMORY INFORMATION:
mem_wire: 101527552 ( 96MB) [ 1%] Wired: disabled for paging out
mem_active: + 94298112 ( 89MB) [ 1%] Active: recently referenced
mem_inactive:+ 68116480 ( 64MB) [ 1%] Inactive: recently not referenced
mem_cache: + 0 ( 0MB) [ 0%] Cached: almost avail. for allocation
mem_free: + 6016483328 ( 5737MB) [ 95%] Free: fully available for allocation
mem_gap_vm: + 8192 ( 0MB) [ 0%] Memory gap: UNKNOWN
-------------- ------------ ----------- ------
mem_all: = 6280433664 ( 5989MB) [100%] Total real memory managed
mem_gap_sys: + -4162113536 ( -3969MB) Memory gap: Kernel?!
-------------- ------------ -----------
mem_phys: = 2118320128 ( 2020MB) Total real memory available
mem_gap_hw: + 29163520 ( 27MB) Memory gap: Segment Mappings?!
-------------- ------------ -----------
mem_hw: = 2147483648 ( 2048MB) Total real memory installed

SYSTEM MEMORY SUMMARY:
mem_used: -3937116160 ( -3754MB) [-183%] Logically used memory
mem_avail: + 6084599808 ( 5802MB) [283%] Logically available memory
-------------- ------------ ----------- ------
mem_total: = 2147483648 ( 2048MB) [100%] Logically total memory


What could be the reason for the negative memory gap ?

This is FREEBSD 10 stable snapshot.

Dmesg shows

real memory = 6442450944 (6144 MB)
Physical memory chunk(s):
avail memory = 6275878912 (5985 MB)
VESA: v3.0, 16384k memory, flags:0x1, mode table:0xea399044 (1000044)
mem: <memory>
hpet0: <High Precision Event Timer> iomem 0xfed00000-0xfed003ff on acpi0

As you can see only 2gb is usable even after enabling pae but dmesg shows all 6gb memory.