为什么这里不用{{}}，而是用{}？？？

Question

<form action="/contest/vacation-photo/{year}/{month}" class="form-horizontal" role="form" enctype="multipart/form-data" method="POST">
    <div class="form-group">
        <label for="fieldName" class="col-sm-2 control-label">Name</label>
        <div class="col-sm-4">
            <input type="text" class="form-control" id="fieldName" name="name">
        </div>
    </div>
    <div class="form-group">
        <label for="fieldEmail" class="col-sm-2 control-label">Email</label>
        <div class="col-sm-4">
            <input type="text" class="form-control" id="fieldEmail" name="email" required>
        </div>
    </div>
    <div class="form-group">
        <label for="fieldPhoto" class="col-sm-2 control-label">Vacation photo</label>
        <div class="col-sm-4">
           
        </div>
    </div>
    <div class="form-group">
        <div class="col-sm-offset-2 col-sm-4">
            <button type="submit" class="btn btn-primary">Submit</button>
        </div>
    </div>
</form>

请问：上述代码中的第一行，form中的： action="/contest/vacation-photo/{year}/{month}"
上述使用handlebars的模板引擎，照理说应该使用{{}}，为什么这里用{}？？？？