svcutil generating MessageContractAttribute or OperationContractAttribute for different services

I'm using svcutil to make proxy classes and I've noticed that for different services I get different results.

I'm using this svcutil command:

svcutil http://server/SomeService.asmx 
    /l:c# 
    /syncOnly 
    /out:C:\ISomeService
    /config:C:\ISomeService.config 
    /namespace:*,SomeServiceProxy

Proxy classes for first service generate code like this:

[System.Diagnostics.DebuggerStepThroughAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute("System.Runtime.Serialization", "4.0.0.0")]
[System.Runtime.Serialization.DataContractAttribute(Name="GetFirstService", Namespace="http://othernamespace.com")]
public partial class GetFirstServiceRequest : SomeServiceProxy.ResponseInfoBase
{
    ...
}

And proxy classes for second service generate this:

[System.Diagnostics.DebuggerStepThroughAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute("System.ServiceModel", "4.0.0.0")]
[System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Advanced)]
[System.ServiceModel.MessageContractAttribute(WrapperName = "GetSecondService", WrapperNamespace = "http://somenamespace.com", IsWrapped = true)]
public partial class GetSecondServiceRequest
{
    ...
}

Why is one class marked with MessageContractAttribute and the other with DataContractAttribute? How does svcutil decide to use one or the other, the command is same for both services?

The DataContract format supports only a subset of XML Schema Definition. So it depends on the service metadata: svcutil (actually System.ServiceModel.Description.WsdlImporter which is used by svcutil) tries to generate DataContracts by default. If it encounters a schema that cannot be represented as a DataContract, it generates a MessageContract as fallback.