请教OpenGl ，同样的数据为什么glBegin(GL_TRIANGLES)和glDrawArrays（）结果显示就不一样呢？

调用中需要使用到的数据结构：
typedef struct
{
float x, /* The x component of the vertex position */
y, /* The y component of the vertex position */
z; /* The z component of the vertex position */
} ObjVertex;

typedef struct
{
float x, /* The x component of the normal vector */
y, /* The y component of the normal vector */
z; /* The z component of the normal vector */
} ObjNormal;

typedef struct
{
float u, /* The u parametric texturing co-ordinate */
v; /* The v parametric texturing co-ordinate */
} ObjTexCoord;

typedef struct
{
unsigned int m_aVertexIndices[3],
m_aNormalIndices[3],
m_aTexCoordIndicies[3];
} ObjFace;

typedef struct _ObjMesh
{
ObjVertex *m_aVertexArray;
ObjNormal *m_aNormalArray;
ObjTexCoord *m_aTexCoordArray;
ObjFace *m_aFaces;
unsigned int *m_aIndices;
unsigned int m_iNumberOfVertices,
m_iNumberOfNormals,
m_iNumberOfTexCoords,
m_iNumberOfFaces;
} ObjMesh;

（1）使用glBegin（）语句：
glBegin(GL_TRIANGLES);
for(i=0;i<pMesh->m_iNumberOfFaces;i++)
{
unsigned int j;
ObjFace *pf = &pMesh->m_aFaces[i];

/*
** Draw the polygons with Normals only
*/
for(j=0;j<3;j++)
{
glNormal3f( pMesh->m_aNormalArray[ pf->m_aNormalIndices[j] ].x,
pMesh->m_aNormalArray[ pf->m_aNormalIndices[j] ].y,
pMesh->m_aNormalArray[ pf->m_aNormalIndices[j] ].z);
glVertex3f( pMesh->m_aVertexArray[ pf->m_aVertexIndices[j] ].x,
pMesh->m_aVertexArray[ pf->m_aVertexIndices[j] ].y,
pMesh->m_aVertexArray[ pf->m_aVertexIndices[j] ].z);
}
}
glEnd();

（2）使用glDrawArrays（）语句调用：
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3,GL_FLOAT,0,pMesh->m_aVertexArray);
if(pMesh->m_aNormalArray)
{
glEnableClientState(GL_NORMAL_ARRAY);
glNormalPointer(GL_FLOAT,0,pMesh->m_aNormalArray);
}
if(pMesh->m_aTexCoordArray)
{
glTexCoordPointer(2,GL_FLOAT,0,pMesh->m_aTexCoordArray);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
}
glDrawArrays(GL_TRIANGLES,0,pMesh->m_iNumberOfFaces);

相同的数据*pMesh，程序运行调用（1）和（2）的显示结果居然不一样。
求解？？？？？？？