Difference in Compressive & Tensile stress values for a symmetrical I beam which is connected to a vertically restrained plated structure and subjected to UDL

Hi,
I was doing a case study for the combined beam & plate effect in STAAD Pro using AISC ASD. 2 geometries were prepared;

1. A simply supported beam subjected to 1kN/m UDL
2.A simply supported beam with plate structure bottom vertically restrained (meshed plate) subjected to 1kN/m UDL.

I have noticed that in geometry 2, the compressive stress and tensile stress are not equal but in 1 the values are same.
What I understood that, this difference in stress is due to the plate below the beam which resist the bending of the bottom flange.

Could someone please suggest on this to get a better explanations.

The model is shown below.

STAAD PLANE
START JOB INFORMATION
ENGINEER DATE 27-Jun-22
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 2.5 0 0; 3 0 -3 0; 4 2.5 -3 0; 5 0.5 0 0; 6 0.5 -0.5 0; 7 0 -0.5 0;
8 1 0 0; 9 1 -0.5 0; 10 1.5 0 0; 11 1.5 -0.5 0; 12 2 0 0; 13 2 -0.5 0;
14 2.5 -0.5 0; 15 0.5 -1 0; 16 0 -1 0; 17 1 -1 0; 18 1.5 -1 0; 19 2 -1 0;
20 2.5 -1 0; 21 0.5 -1.5 0; 22 0 -1.5 0; 23 1 -1.5 0; 24 1.5 -1.5 0;
25 2 -1.5 0; 26 2.5 -1.5 0; 27 0.5 -2 0; 28 0 -2 0; 29 1 -2 0; 30 1.5 -2 0;
31 2 -2 0; 32 2.5 -2 0; 33 0.5 -2.5 0; 34 0 -2.5 0; 35 1 -2.5 0; 36 1.5 -2.5 0;
37 2 -2.5 0; 38 2.5 -2.5 0; 39 0.5 -3 0; 40 1 -3 0; 41 1.5 -3 0; 42 2 -3 0;
43 5 0 0; 44 5.5 0 0; 45 6 0 0; 46 6.5 0 0; 47 7 0 0; 48 7.5 0 0;
MEMBER INCIDENCES
1 1 5; 3 5 8; 5 8 10; 7 10 12; 9 12 2; 37 43 44; 38 44 45; 39 45 46; 40 46 47;
41 47 48;
ELEMENT INCIDENCES SHELL
4 1 5 6 7; 6 5 8 9 6; 8 8 10 11 9; 10 10 12 13 11; 11 12 2 14 13; 12 7 6 15 16;
13 6 9 17 15; 14 9 11 18 17; 15 11 13 19 18; 16 13 14 20 19; 17 16 15 21 22;
18 15 17 23 21; 19 17 18 24 23; 20 18 19 25 24; 21 19 20 26 25; 22 22 21 27 28;
23 21 23 29 27; 24 23 24 30 29; 25 24 25 31 30; 26 25 26 32 31; 27 28 27 33 34;
28 27 29 35 33; 29 29 30 36 35; 30 30 31 37 36; 31 31 32 38 37; 32 34 33 39 3;
33 33 35 40 39; 34 35 36 41 40; 35 36 37 42 41; 36 37 38 4 42;
ELEMENT PROPERTY
4 6 8 10 TO 36 THICKNESS 0.015
DEFINE MATERIAL START
ISOTROPIC DUMMY
E 20500
POISSON 0.3
DENSITY 76.819
ISOTROPIC STEEL
E 2.05e+008
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-005
DAMP 0.03
TYPE STEEL
STRENGTH FY 253200 FU 407800 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 3 5 7 9 37 TO 41 TAPERED 0.5 0.015 0.5 0.3 0.02 0.3 0.02
CONSTANTS
MATERIAL STEEL ALL
SUPPORTS
1 2 43 48 PINNED
3 4 39 TO 42 FIXED BUT FX FZ MX MY MZ
LOAD 1 LOADTYPE None TITLE LOAD CASE 1
MEMBER LOAD
1 3 5 7 9 37 TO 41 UNI GY -1
PERFORM ANALYSIS PRINT ALL
PRINT MEMBER INFORMATION ALL
PRINT MEMBER INFORMATION ALL
PARAMETER 1
CODE AISC
LZ 2.5 ALL
TRACK 2 ALL
CHECK CODE ALL
FINISH