strange gas volume factor (Bg) in gas condensate field

I am looking at a gas condensate e300 model.
I calculated Bgi using PRT data

reservoir vol of gas / reservoir volume gas at surface cond

Bg look very strange
more than that there are time steps when i see increasing of reservoir volume gas at surface cond
how is this possible?
reservoir volume gas at surface cond should not decrease monotonicaly with time ?

e x a m p l e

first time step
Fluid in place totals
---------------------

Average pressure PV weighted: 396.315 Barsa
HCPV weighted: 367.552 Barsa
Reservoir volume of gas 103.965161 M RM3

With respect to separator
-------------------------

Surface volume Moles
------------------------------ ------------------------------
Oil 18.198637 M SM3 113.265346 M Kg-M
Gas 24499.240259 M SM3 1034.069552 M Kg-M

third time step

Fluid in place totals
---------------------

Average pressure PV weighted: 396.315 Barsa
HCPV weighted: 367.552 Barsa

Reservoir volume of gas 103.965161 M RM3

With respect to separator
-------------------------

Surface volume Moles
------------------------------ ------------------------------
Oil 20.429989 M SM3 127.152913 M Kg-M
Gas 27503.114458 M SM3 1160.857763 M Kg-M