新手求教，用beam189计算简支梁重力作用下弯矩为何偏大

如题，命令流如下：
/prep7
k,1
k,2,5
l,1,2
mp,ex,1,2e11
mp,gxy,1,7.7e10
mp,dens,1,7800
mp,prxy,1,0.3
et,1,beam189
sectype,1,beam,rect
secdata,0.2,0.05
k,100,2.5,100
latt,1,1,1,,100,,1
lesize,all,,,6
lmesh,all
finish
/solu
d,1,ux,,,,,uy,uz
d,2,uy,,,,,uz
acel,,9.8
solve
/post1
etable,mi,smisc,2
etable,mj,smisc,15
plls,mi,mj
计算最大弯矩为2432.99，和理论值2388.75有差别。大家帮忙看看是什么原因。谢谢！