What makes entry.softTtl and entry.ttl different from volleyball?

Question

What makes entry.softTtl and entry.ttl different from volleyball?

In the HttpHeaderParser class:

public static Cache.Entry parseCacheHeaders(NetworkResponse response) {
    long now = System.currentTimeMillis();

    Map<String, String> headers = response.headers;

    long serverDate = 0;
    long serverExpires = 0;
    long softExpire = 0;
    long maxAge = 0;
    boolean hasCacheControl = false;

    String serverEtag = null;
    String headerValue;

    headerValue = headers.get("Date");
    if (headerValue != null) {
        serverDate = parseDateAsEpoch(headerValue);
    }

    headerValue = headers.get("Cache-Control");
    if (headerValue != null) {
        hasCacheControl = true;
        String[] tokens = headerValue.split(",");
        for (int i = 0; i < tokens.length; i++) {
            String token = tokens[i].trim();
            if (token.equals("no-cache") || token.equals("no-store")) {
                return null;
            } else if (token.startsWith("max-age=")) {
                try {
                    maxAge = Long.parseLong(token.substring(8));
                } catch (Exception e) {
                }
            } else if (token.equals("must-revalidate") || token.equals("proxy-revalidate")) {
                maxAge = 0;
            }
        }
    }

    headerValue = headers.get("Expires");
    if (headerValue != null) {
        serverExpires = parseDateAsEpoch(headerValue);
    }

    serverEtag = headers.get("ETag");

    // Cache-Control takes precedence over an Expires header, even if both exist and Expires
    // is more restrictive.
    if (hasCacheControl) {
        softExpire = now + maxAge * 1000;
    } else if (serverDate > 0 && serverExpires >= serverDate) {
        // Default semantic for Expire header in HTTP specification is softExpire.
        softExpire = now + (serverExpires - serverDate);
    }

    Cache.Entry entry = new Cache.Entry();
    entry.data = response.data;
    entry.etag = serverEtag;
    entry.softTtl = softExpire;
    entry.ttl = entry.softTtl;
    entry.serverDate = serverDate;
    entry.responseHeaders = headers;

    return entry;
}

entry.softTtl = softExpire;
entry.ttl = entry.softTtl;

These two variables have the same meaning, so why?

In class CacheDispatcher

@Override
public void run() {
    ...
    ...

    // If it is completely expired, just send it to the network.
    if (entry.isExpired()) {
        request.addMarker("cache-hit-expired");
        request.setCacheEntry(entry);
        mNetworkQueue.put(request);
        continue;
    }

    ...

    if (!entry.refreshNeeded()) {
        // Completely unexpired cache hit. Just deliver the response.
        mDelivery.postResponse(request, response);
    } else {
        ...

        // Post the intermediate response back to the user and have
        // the delivery then forward the request along to the network.
        mDelivery.postResponse(request, response, new Runnable() {
            @Override
            public void run() {
                try {
                    mNetworkQueue.put(request);
                } catch (InterruptedException e) {
                    // Not much we can do about this.
                }
            }
        });
    }

    ...
}